Oxygen inhibits radical polymerization, and for the present investigation, the thickness of the resulting unpolymerized surface layer on various proprietary dental polymers was measured by a microscopic technique. The inhibition depth of the polymerized resins varied from 7 to 84μm. Resin systems with the tertiary aromatic amine 3, 4-xylyldiethanolamine as the activator had a thinner unpolymerized layer than those with p-tolyldiethanolamine. Increased viscosity also resulted in reduced thickness of the unpolymerized films. UV-light cured resins had thinner inhibited layers than those of comparable viscosity with a peroxide-amine initiator system. However, the thinnest unpolymerized film was seen with a chemically activated resin system containing acetone.This investigation has shown that the thickness of the unpolymerized film on cured dental resins is related to the composition and the initiating system.