SLES vs SLS: Which Surfactant Belongs in Your Formula

Every wash formulation eventually runs into this fork in the road. SLES and SLS look almost identical on paper, one letter apart, both cheap, both foam like crazy. But they behave differently on skin, they carry different manufacturing baggage, and mixing them up on a formula sheet is an easy way to end up with a harsher product than you intended.

What they actually are

SLS (Sodium Lauryl Sulfate), INCI name Sodium Lauryl Sulfate, CAS 151-21-3, is a sulfated fatty alcohol, meaning it's made by reacting lauryl alcohol directly with a sulfating agent. It's a strong anionic surfactant with excellent foaming and cleansing power.

SLES (Sodium Laureth Sulfate), INCI name Sodium Laureth Sulfate, CAS 9004-82-4 (this one is often listed as a CAS number covering a range of ethoxylated products rather than one single fixed structure), is made the same way but the lauryl alcohol is first ethoxylated, meaning ethylene oxide units are added to the chain before sulfation. That "eth" in the name is your tell: ethoxylated.

Why the ethoxylation matters

Adding those ethylene oxide units changes the molecule's behavior in three practical ways:

• Mildness. SLES is generally considered gentler on skin and eyes than SLS. This is the main reason SLES largely replaced SLS in leave-longer products like shampoo and body wash, while SLS still shows up in shorter-contact applications or where cost and raw foaming power matter more than mildness.
• Water solubility and thickening behavior. SLES responds more predictably to salt for viscosity building in liquid formulas, which is part of why it's the surfactant most home and small-batch liquid soap and shampoo makers reach for when they need a specific gel thickness.
• Foam texture. SLS tends to produce a tighter, denser foam. SLES produces a looser, more voluminous foam that many consumers associate with "shampoo lather."

The 1,4-dioxane question

This is the part that generates the most anxious emails. The ethoxylation process used to make SLES can produce 1,4-dioxane as a processing byproduct if the reaction isn't properly controlled or the product isn't adequately vacuum-stripped afterward. SLS, because it isn't ethoxylated, doesn't carry this same processing risk.

A few things worth knowing here:

1. This is a manufacturing process consideration, not an inherent property of the INCI ingredient itself. A well-processed, properly stripped SLES should have negligible dioxane content.
2. It's exactly why supplier documentation matters. Ask your raw material supplier for their specification on 1,4-dioxane content and keep that certificate of analysis on file.
3. This is a case where your safety data sheet, particularly the section on composition and any known impurities, is worth actually reading rather than filing away unopened.

Side-by-side comparison

Attribute	SLS	SLES
INCI name	Sodium Lauryl Sulfate	Sodium Laureth Sulfate
CAS number	151-21-3	9004-82-4 (range, ethoxylated)
Relative mildness	Harsher	Milder
Foam character	Dense, tight	Voluminous, loose
Salt-responsive thickening	Less predictable	More predictable
Processing byproduct concern	Not applicable	1,4-dioxane (process-dependent)
Typical use	Shorter-contact cleansers, industrial	Shampoo, body wash, liquid soap

What this means for your notification

Whichever one you use, or if you use both in the same formula, each needs its own line on your ingredient declaration with its own INCI name and concentration. Don't fold them together as "sulfate surfactants." A reviewer, or a notification system, needs the exact INCI name to check it against the restricted and prohibited substance lists, and SLS and SLES are treated as distinct ingredients with distinct entries.

If you're formulating for Canada, remember the notification itself asks for the INCI name and a concentration or concentration range, not a brand name and not "surfactant blend." If you bought a pre-made surfactant blend from a supplier that includes SLES along with a co-surfactant like cocamidopropyl betaine, you need to break that blend into its actual INCI components and calculate the real percentage of each in your finished product, which means multiplying the blend's use level in your recipe by each component's share of the blend.

Sorting out exactly which surfactants and blends end up on a filing, at what real concentration, is the kind of detail work Cosmetic Comply was built to catch. It expands supplier blends into their true INCI components, carries the math through, and screens the result before you submit.