Thiol-ene photopolymerizations without added photoinitiators were studied, and the kinetics accurately predicted by modeling for a wide range of different vinyl chemistries. Initiation rates in polymerizations initiated with light centered around 365 nm were found to be proportional to the concentration of ene functional groups. When 254 nm light was used as the initiating light source, initiation rates were proportional to the concentration of thiol functional groups. The mechanism of initiation at 254 nm was attributed to direct cleavage of thiol functional groups. An appropriate species or mechanism has not yet been found that is consistent with the experimental data for initiation when 365 nm light is used. (C) 2004 Wiley Periodicals, Inc.