In situ mass spectrometry is demonstrated as a technique to study the pyrolysis of neat fluids at supercritical conditions. These fluids included pure hexane, benzene, and binary mixtures of the two, which were sampled in a supersonic expansion to cool and trap reactants, intermediates, and products in a molecular beam. To identify the reacting species, the molecular beam was subjected to electron impact ionization prior to analysis in a quadrupole mass filter. In addition to the previously reported gas-phase pyrolysis products for hexane and benzene, we observe the enhanced production of biphenyl in the binary mixture, which can be attributed to an energetically favored pathway by which the initial production of alkyl radicals seeds the formation of phenyl radicals via H atom abstraction reactions. These phenyl radicals quickly react to form biphenyl because of the proximity of solvent reaction partners and high collision frequency in the fluid. Our results illustrate a simple model system that highlights contemporary difficulties associated with multicomponent fuels due to the new pathways that become available even in simple mixtures.