Is ethoxydiglycol an active ingredient?

No. It is a solvent. It dissolves and it carries; it does nothing to skin on its own, and no amount of it is a benefit in itself.

Is it the same as diethylene glycol?

No — it is a monoethyl ether of it, a different molecule. The real question is the purity of the grade used, which is a raw-material specification and not something an ingredient list reports.

Why does my serum with it in sting?

Solvent-heavy formulas often do, especially on skin that has just been exfoliated or is already irritated. If it stings past the first minute or two, that formula is not for your skin right now.

Does it make actives work better?

It can make more of them available, which is not the same claim. It makes more of everything else available too, so read a formula built on it as stronger than its percentage suggests.

There is no general reason to. It is one of the ordinary tools that makes a difficult active formulable at all. Sensitive or compromised skin may prefer formulas that lean on it less.

Ethoxydiglycol — Ingredient Guide

How it works

Most cosmetic ingredients fall neatly on one side or the other: they dissolve in water, or they dissolve in oil. The interesting actives have a habit of doing neither well. Salicylic acid, several of the resorcinols, some UV filters and a number of the vitamin derivatives are stubbornly hard to keep in solution, and a molecule that has crystallised out of a product has stopped being an ingredient and started being grit.

Ethoxydiglycol is a small glycol ether that mixes with water and with a good many oils, so it can bridge the two. Dissolve a difficult active in it first and the active will stay dissolved in the finished formula, at a level that would otherwise need an unpleasant amount of alcohol to reach.

The evidence

Its function is a solubility one and it is measured that way: what dissolves, how much of it, and whether it is still dissolved after a month in a warm room. That is a real and useful property, and it says nothing about skin.

The second property is the one to be careful with. A solvent comfortable in both phases also changes how an active divides between the vehicle and the skin, and ethoxydiglycol is often chosen with that in mind. Enhancement is not selective, though. A vehicle that helps the active in is helping everything else in too, which is one reason a well-made and a badly made formula with the same ingredient list can behave very differently.

Suitability

Not an active, and nothing in a routine depends on it
Formulas leaning heavily on it can sting on freshly exfoliated or compromised skin
Its name sits close to diethylene glycol; they are different molecules, and the practical question is raw-material purity, which is a supplier specification rather than something a label answers
Common in serums and spot treatments built around a poorly soluble active — its presence usually says the formula had a solubility problem to solve

Concentration

No figure is quoted. The working level is set by how much active there is to dissolve rather than by anything about skin, and there is no level at which it does something for you. How much may be used is market-specific and gets revised, so the current local rule is the thing to check rather than a number on a page.

Conflicts & combinations

Nothing conflicts with it chemically. The combination worth thinking about is the one it makes possible: a product pairing it with a strong active is delivering that active more efficiently than the percentage alone suggests, which is an argument for introducing that product slowly rather than for avoiding it.