The mechanism for the quenching by oxygen of the lowest triplet state (T-1) of 9-acetylanthracene(ACA) in five saturated hydrocarbon solvents at pressures up to 400 MPa was investigated. The quenching rate constants of the T1 state of ACA, k(q)(T), decrease monotonically with increasing pressure, and the apparent activation volumes for k(q)(T) vary in the range 0.8-5.3 cm(3)/mol. It was also found that the plots of In k(q)(T) against In eta, where eta is solvent viscosity, show significant downward curvatures in all the solvents examined. From measurements by the time-resolved thermal lensing at 0.1 MPa, together with measurements of triplet-triplet absorption spectra as a function of pressure, the yields of the T1 state of ACA were found to be approximately unity in the experimental conditions examined. The quenching rate constants, k(q)(S), by oxygen of the lowest excited singlet state (S-1) of 9,10-dimethylanthracene (DMEA) whose van der Waals radius is nearly equal to that of ACA decrease strongly with increasing pressure, and the apparent activation volumes for k(q)(S) fall in the range of 9.4-14.9 cm(3)/mol. It was also found that the plots of In k(q)(S) against -ln eta are linear, with a slope of 0.59-0.71 depending on solvent. These results of k(q)(S) are consistent with our previous conclusion that the oxygen quenching of the S-1 state of DMEA is diffusion controlled. The ratio, k(q)T/k(q)(S), is approximately 1/9 in methylcyclohexane but is less than 1/9 in n-butane, n-pentane, n-hexane, and n-heptane at 0.1 MPa and 25 degrees C, and the ratio was found to increase over 1/9 with increasing pressure in all the solvents examined. By the bleaching method of DPBF, coupled with time-resolved luminescence measurements, the yields of singlet oxygen ((1)Delta(g)) formed by the quenching of the T-1 state of ACA, Phi(Delta), were measured, and the values of Phi(Delta) were found to be approximately unity. These results were explained by a kinetic model in which the intersystem crossings between encounter complexes with different spin multiplicities are taken into account. From the analysis based on this model, the pressure dependence of k(q)T/k(q)S is discussed.