The quenching by oxygen of the lowest electronically excited singlet (S 1) and triplet (T 1) states of five aromatic molecules in methylcyclohexane (MCH) at pressures up to 400 MPa was investigated. The apparent activation volume for the S-1 state, DeltaV(q)(Sdouble dagger) at 0.1 MPa fell in the range from 14 to 16 cm(3)/mol, which is significantly smaller than DeltaV(eta)(double dagger) (25 cm(3)/mol) determined from the pressure dependence of the solvent viscosity, eta, whereas that for the T-1 state, DeltaV(q)(Tdouble dagger), at 0.1 MPa changed from +6.1 (anthracene) to -15.2 cm(3)/mol (triphenylene) and correlated approximately linearly with the triplet energy, E-T. However, DeltaV(q)(Tdouble dagger) at 350 MPa was positive and almost independent of E-T (3-6 cm(3)/mol). The pressure dependence of k(q)(S) was interpreted in the framework reported previously by us, and that of k(q)(T) was attributed to the contribution of diffusion to the quenching in which the encounter complex pair with singlet, triplet, and quintet spin multiplicities formed between the oxygen and T-1 state molecules is involved.