Anionic polymerization has been successfully employed for reactive processing, the most notable example being that of nylon RIM [1] using caprolactam polymerization. Here, fast polymerization is achieved with the help of a combination of a catalyst and an initiator under practically anhydrous conditions. This polymerization method leads to a narrow molecular weight distribution and a Poisson distribution of the individual chain lengths. Analytical expressions for calculating the number-average and weight-average degrees of polymerization of the polymer chains from the degree of conversion have been derived based on this knowledge [2]. But in a reactive system where a substantial amount of monomer may be present during most of the processing time, the average must include the monomer units present. Only such averages will be of use in correlations of molecular weight with viscosity and degree of conversion with viscosity, which are of importance in the design of a reactive processing system. A method to compute such average molecular weights is presented here.