The contribution of diffusion to the fluorescence quenching by oxygen of 9, 10-dimethylanthracene (DMEA) in liquid CO2 and supercritical CO2 (SCF CO2) at pressures up to 60 MPa was investigated. For comparison, the fluorescence quenching by CBr4 of DMEA was also investigated. The apparent activation volume of the quenching rate constant, k(q), was 8 +/- 1 and 10 +/- 3 cm(3)/mol for DMEA/O-2, and 42 +/- 7 and 400 +/- 90 cm(3)/mol for DMEA/CBr4 in liquid CO2 (25 degreesC, 10 MPa) and SCF CO2 (35 degreesC, 8.5 MPa), respectively. For DMEA/O-2, the plots of In kg against In eta, where eta is the solvent viscosity, showed a leveling-off with decreasing the solvent viscosity, whereas for DMEA/CBr4 they were almost linear in both liquid and SCF CO2. The results, together with those of the pressure and the pressure-induced solvent viscosity dependences of k(q) for DMEA/O-2 in n-alkanes (C-4 to C-7) and for DMEA/CBr4 in n-hexane, revealed that the quenching competes with diffusion. The contribution of diffusion to the quenching was analyzed on the basis of a kinetic model with solvent cage in which the quenching occurs. The bimolecular rate constant for the quenching in the solvent cage, k(bim). was 6.0 X 10(10) and 12 x 10(10) M-1 s(-1) in liquid CO2 (25 degreesC, 10 MPa) for DMEA/O-2 and DMEA/CBr4, respectively, and 5.7 x 10(10) and 12 x 10(10) M-1 s(-1) in SCF CO2 (35 degreesC, 8.5 MPa) for DMEA/O-2 and DMEA/CBr4. respectively. The pressure dependence of k(bim) and the contribution of diffusion to the quenching are discussed.