Pulsed supersonic, plasma sampling mass spectrometric measurements of the concentrations of the molecular species present in 2% acetylene in deuterium electron cyclotron resonance-microwave plasmas have been compared to the concentrations observed in analogous plasmas based exclusively on hydrogen. In the hydrogen plasmas, the molecular composition is found to be similar to 76% acetylene. In the deuterium plasmas, unlabeled, singly deuterated and doubly deuterated acetylene are all observed with a relative abundance of similar to 1:3:5, respectively. A simple statistical analysis of the observed isotopic ratios leads to the conclusion that the distribution of molecular species in the plasma environment is the result of an average of only six reactive collisions.