Butylene Glycol vs Propylene Glycol: Picking a Humectant Solvent

A formulator asked me recently why her toner felt slightly tacky compared to a reference product that used what looked like an almost identical ingredient list. The difference turned out to be one glycol swapped for another. Propylene glycol and butylene glycol get treated as interchangeable more often than they should be, and the gap between them shows up in texture, in how well they carry other ingredients into solution, and eventually in what actually lands on your CNF.

What these ingredients are doing in your formula

Both are small polyol molecules used as humectants, meaning they help draw and hold moisture, and both double as solvents that help dissolve things like plant extracts, some actives, and fragrance components that would otherwise sit awkwardly in a water-based formula. That overlapping job description is exactly why they get treated as swappable. The differences show up once you look closer.

Butylene Glycol

• INCI name: Butylene Glycol
• CAS number: 107-88-0 (note some sources list a related CAS for a specific isomer; confirm the exact form your supplier is providing)
• Typical function: humectant, solvent, viscosity-reducing agent
• Feel on skin: generally lighter, less sticky, often preferred in serums and lightweight lotions where a "dry-down" feel matters
• Solubilizing strength: strong solvent for many botanical extracts and some fragrance materials, often used specifically to help plant extracts go into solution cleanly

Propylene Glycol

• INCI name: Propylene Glycol
• CAS number: 57-55-6
• Typical function: humectant, solvent, penetration enhancer in some formulations
• Feel on skin: can read as slightly tackier at higher use levels compared to butylene glycol, though this is formulation-dependent and not universal
• Solubilizing strength: also a strong solvent, widely used historically and still common in both cosmetic and pharmaceutical contexts

Side by side

Attribute	Butylene Glycol	Propylene Glycol
INCI name	Butylene Glycol	Propylene Glycol
CAS number	107-88-0	57-55-6
Primary roles	Humectant, solvent	Humectant, solvent, penetration enhancer
Typical skin feel	Lighter, less tacky	Can feel tackier at higher levels
Common use case	Serums, essences, lightweight lotions	Broader range including heavier creams and some functional formulations
Extract solubilizing	Frequently chosen specifically for this	Also effective, longer track record in the category

Why formulators lean one way or the other

If you're building a lightweight, fast-absorbing product and the tackiness of propylene glycol at your target concentration bothers you in testing, butylene glycol is the more common swap, and it often lets you hit a similar humectant and solvency profile without the same skin feel tradeoff. If cost matters more than a slight feel difference, propylene glycol tends to be the more budget-friendly option and has a long history of use across both cosmetic and pharmaceutical formulations, which some brands find reassuring from a track record standpoint.

Neither one is "safer" or "riskier" than the other in a way that should drive the decision on its own. This is largely a formulation and sensory choice, not a compliance one, though of course actual concentration in your finished product still needs to be tracked accurately regardless of which you pick.

What this means for your filing

Whichever one you use, and however you obtained it, whether as a standalone raw material or as part of a supplier blend, the INCI name that goes on your Cosmetic Notification Form has to be exact. "Butylene Glycol" and "Propylene Glycol" are not interchangeable on paper any more than they are in your beaker, and a filing that lists the wrong one because someone copied a trade name sheet without checking the actual INCI mapping is a mistake that can surface later during any review of your notification.

If either glycol arrives to you as part of a supplier's proprietary blend, remember the concentration math: take the blend's percentage in your finished formula and multiply it by the glycol's percentage within that blend to get the real finished-product concentration. A blend used at 3% that is itself 40% butylene glycol puts the actual finished-product concentration at 1.2%, not 3%, and that's the number that belongs on your filing.

This is exactly the kind of ingredient-mapping and blend-expansion work Cosmetic Comply handles automatically, matching whatever name your supplier gave you to the correct INCI name and CAS number, carrying blend percentages through to real concentrations, and screening the result against the Cosmetic Ingredient Hotlist before the CNF gets filed.