Online size exclusion chromatography–electrospray ionization–mass spectrometry (SEC/ESI–MS) is employed for quantifying the overall initiation efficiencies of photolytically generated radical fragments. In a unique experiment, we present the first quantitative and systematic study of methyl‐substituted acetophenone‐type photoinitiators being employed in a single cocktail to initiate the free‐radical polymerization of methyl methacrylate (MMA) in bulk. The photoinitiators are constituted of a set of two known and four new molecules, which represent an increasing number of methyl substituents on their benzoyl fragment, that is, benzoin, 4‐methylbenzoin, 2,4‐dimethylbenzoin, 2,4,6‐trimethylbenzoin, 2,3,5,6‐tetramethylbenzoin, and 2,3,4,5,6‐pentamethylbenzoin. The absolute quantitative evaluation of the mass spectra shows a clear difference in the initiation ability of the differently substituted benzoyl‐type radical fragments: Increasing the number of methyl substituents leads to a decrease in incorporation of the radical fragments.