The effect of solvent viscosity on the quenching by oxygen of S1 and T1 states was investigated for a number of meso-substituted anthracene derivatives. The solvent viscosity, r], was varied by applying hydrostatic pressure at several temperatures. The fluorescence from the S1 state of the anthracene derivatives that have one or two electron-donating substituents was quenched nearly collisionally, and the dependence of the rate constant for fluorescence quenching, kS, on r] was shown to be described satisfactorily by the empirical function, Aq". It was shown for 9-methylanhracene in methylcyclohexane that the exponent, a, has a value of 0.68 f 0.06, which is independent of temperature and pressure but that thevalue of the constant of proportionality, A, is temperature dependent. The dependence of A on temperature was described empirically using the functions P.36 or exp(-AEIRT). An alternative function for is (T/r])@, but this function failed to describe correctly the dependence of kt on 9 and T. The fractional power dependence of k: on 4 suggests that the rate constant for the intrinsic fluorescence quenching reaction, kaCt, is not significantly larger than the rate constant for diffusion.The values of k: for the anthracene derivatives containing one or two electron-attracting substituents were smaller than that found for anthracene, decreased with Hammett's up parameter, and could not be described by the empirical function A?* with a constant value for a. A typical example is 9,lO-dicyanoanthracene whose values for k: exhibit a convex dependence on In r]. In accordance with this, the activation energy associated with k: was negative at low viscosities and displayed a transition from negative to positive values as the viscosity was increased. The values of the rate constant for the quenching by oxygen of the T1 states of the anthracene derivatives, k t , did not vary significantly, all being approximately 3 X lo9 M-1 s-l at 25 OC and 0.1 MPa. The logarithmic values of k;, when plotted as a function of In r], displayed appreciable downward curvature for all of the anthracene derivatives examined in this work.