The product distributions for the photochemical electron transfer and triplet-triplet energy-transfer-induced dimerization of 1,3-cyclohexadiene (CHD) were determined as a function of pressure and solvent in the range of 0.1-203 MPa. In the case of the photochemically (dicyanonaphthalene sensitized) induced electron-transfer dimerization, differences in activation volumes for the formation of endo-1 and exo-2 adduct are small and positive in acetonitrile (ca. +1 to +2 cm3/mol) but are negative and unusually large (ca. -9 to -12 cm3/mol) in benzene. The results are consistent with the involvement of different types of solvated ion pairs in the two solvents. Although the product distribution does not change for any of the sensitizers and pressures used in the triplet-sensitized reaction, the efficiency of dimerization was surprisingly both pressure and solvent dependent. The activation volumes for efficiency of dimerization of CHD by triplet-triplet energy transfer fall in the range -22 to +2.1 cm3/mol and depend on both the triplet energy of the sensitizer and the solvent used.