For the first time, organic electrochemical processes in aprotic media were studied by means of cyclic voltammetry at pressures of up to 300 MPa. The measurements at different concentrations, scan rates, temperatures, and pressures provide evidence of the reversible dimerization of electrochemically generated radical anions of trinitrotoluene, naphthalenedicarbonitrile, 9-cyanoanthracene, and radical cations of triphenylamine. The reaction volumes of the dimerization of the substances studied were determined as -36, -19, -35, and -23 cm(3) mol(-1), respectively. Their negative values manifest the shift of the equilibrium of the dimerization under high pressure toward the formation of dimers. The small difference between the respective reaction volumes in acetonitrile and dichloromethane points to the dominance of the intrinsic geometry changes over solvation effects in this process.