A program was developed that yields the number of chain scission and crosslinking events representing the best fit of a simulated molar mass distribution to a molar mass distribution determined by size exclusion chromatography. The program was applied to molar mass distributions of samples taken from pressure tested pipes of an unstabilized medium-density polyethylene. Good fits were obtained after various exposure times at different temperatures and at different positions in the pipe wall. The ratio of the number of chain scission to crosslinking events generally decreased with increasing exposure time and with decreasing accessibility to oxygen in the pipe wall. Correction for long-chain branching effects led to significant changes in the optimum chain scission and crosslinking values obtained. The broad and asymmetric shape of the molar mass distributions, typical of degraded material from originally stabilized medium-density pipes, was shown to arise from a combination of a uniformly degraded and a nondegraded component. The program was also successfully applied to the molar mass distribution of a sample taken from a degraded commercial polybutylene-1 pipe.