The bimolecular rate constant, k(TT), for triplet-triplet annihilation (TTA) of 2-acetonaphthone (ACN) in methylcyclohexane (MCH) and n-hexane at pressures up to 400 MPa at 25 degreesC has been measured. It was found that k(TT) decreases significantly with increasing pressure, and the activation volumes for k(TT), DeltaV(TT)(double dagger) were determined to be 18 and Il cm(3)/mol, whereas those for the solvent viscosity, DeltaV(eta)(double dagger), were 24 and 23 cm3/mol in MCH and n-hexane, respectively. The significant difference between DeltaV(TT)(double dagger) and DeltaV(eta)(double dagger) was attributed to the competition of the quenching with diffusion. From the analysis on the basis of the pressure dependence of the solvent viscosity, eta, On the quenching, the observed rate constant, k(TT), was separated into the contributions of the sate constant for diffusion, k(diff), and the bimolecular rate constant for quenching, k(bim), in the solvent cage. By using the values of k(diff) thus determined, 2k(TT)/k(diff) was found to be about 4/9 at 0.1 MPa End to approach unity with increasing pressure. The results were interpreted by a kinetic model that involves the encounter complex pairs, (i)(M*M*)(en) (i = 1, 3, 5), with singlet, tripler, and quintet spin multiplicities. It was concluded that the rate constant for the dissociation, k-diff, decreases more significantly than that for the intersystem crossing, k(en)isc(i), between the encounter complex pairs with increasing pressure (Scheme 4).