The role of silanes, germanes, and stannanes R3XH as co-initiators and additives to overcome the oxygen inhibition usually encountered in photopolymerization reactions in aerated media is demonstrated in experimental conditions where both a low light intensity and thin and low-viscosity samples are used. These results concerned with silanes as co-initiators are compared to data previously obtained in more viscous media where a lower effect of oxygen is expected. The striking role of R3XH here is their use as additives in both type I and II photoinitiating systems. It allows to dramatically increase the polymerization rates and the final monomer conversions in the above-selected conditions. The mechanisms investigated by laser flash photolysis (LFP) and electron spin resonance (ESR) experiments are discussed.