The attachment of electrons to pyrazine occurs reversibly over a wide range of pressures at and above room temperature in supercritical xenon. The rate constant for attachment increases with pressure at low pressures, passes through a maximum, and levels off at values of 1-3 x 10(12) m(-1) s(-1) at high pressure. The activation volumes for attachment (Delta V-a(*)) are quite small but show maxima near the compressibility maxima. In contrast, Delta V-a(*) is always negative for this reaction in sc-ethane and exhibits minima near the compressibility maxima. The rate constants for electron detachment change little with pressure but increase with temperature. Activation volumes for detachment are small. To explain the small volume change observed for this reaction, it is proposed that at the higher pressures clustering around the neutral pyrazine is comparable to that around the ion; i.e., the partial molar volumes are comparable. The free energy change (Delta G(r)) of this reaction decreases between 40 and 60 bar and then is fairly constant at higher pressures. The dependence of Delta G(r) on pressure is consistent with clustering around the neutral pyrazine at higher pressure. Also, the electron affinity of the clusters, pyrazineXe(n), increases with n to a few tenths of an eV.