A structure-reactivity study on 4-nitro-1-naphthylamine (NNA) and N-acethyl-4-nitro-1-naphthylamine (ANNA), efficient photoinitiators of polymerization in the presence of N,N-dimethylaniline (DMA), was carried out applying time-resolved laser (337 nm) spectroscopy. The laser-induced phosphorescence of these initiators is efficiently quenched by DMA inducing the appearance of new emissions as result of radiationless processes. The spectral and temporal characteristics of these new bands together with their dependence on both the laser intensity and irradiation time allowed their assignation and provided evidence for the photosensitization process of DMA from the excited initiator to be the photoinitiation mechanism of polymerization.