Initiator efficiency is usually between 0.3 and 0.8 at the start of the,polymerization reaction and decreases as the reaction proceeds until it reaches a limiting value of zero. Using 2,2-dimethoxy-2-phenylacetophenone as a model initiator, a theoretical investigation of the initiator efficiency was undertaken. Taking into consideration all the significant reactions that the radicals undergo, expressions were developed that allow for the prediction of the initiator efficiency at the onset and during the course of the polymerization. All the parameters involved in these expressions have physical and fundamental significance. A comparison of the predicted value for the initiator efficiency with an available experimental value showed excellent agreement. The effect of various model parameters on the efficiency prediction is discussed. Comparisons were also made with previous models in an effort to explain some experimental observations that could not be explained by them.