Most studies in theoretical chemistry are interest to model microscopic properties of atoms and molecules that participate of some experiment. In many cases these approaches leading them to a very complex construction and using, in general, stochastic processes to describe the behavior of the system. In this paper we considered a mathematical approach to describe the behavior (variation in time) of chemicals concentration upon the photoinitiated polymerization of the terahydrofuran by systems sensitizer-sulfonium salt. The mathematical approach is based in an ordinary differential equation where the parameters are time-dependent functions, which were solved numerically by Euler Numerical Method, first order. The main properties of the chemicals involved in this reaction were responsible to generate three assumptions used as a basis to construct the present approach. We argue that these assumptions are responsible to model completely the system. This verified in the theoretical results. The ideas presented her can be easily modified to any system where the chemicals that interact with the experimental environment follow the same patterns of behavior. Results showed that these assumptions combined with the correct mathematical procedures resulted in a set of patterns observed experimentally. (c) 2005 Elsevier Ltd. All fights reserved.