The thermodynamics of the equilibrium reaction e(-) + pyrimidine reversible arrow pyrimidine(-) was studied in tetramethylsilane (TMS) and 2,2,4-trimethylpentane (TMP) as solvents. In hexane, only the forward attachment reaction could be observed. The attachment rate constants (k(a)) increase with temperature in all solvents. The equilibrium shifts to the right with increasing pressure because of the large, negative reaction volumes (-200 to -330 cm(3)/mol). The volume changes are consistent with electrostriction by the pyrimidine anion when a glass shell of solvent molecules is included. The free energy of reaction at 298 K is -0.43 eV in tetramethylsilane at 1 bar and -0.70 eV in 2,2,4-trimethylpentane at 150 bar. The solution free energies indicate the gas-phase (adiabatic) electron affinity of pyrimidine is -0.24 eV.