The effect of solvent viscosity on the quenching by oxygen of S-1 and T-1 states was investigated for a number of meso-substituted anthracene derivatives. The solvent viscosity, eta, was varied by applying hydrostatic pressure at several temperatures. The fluorescence from the S-1 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, k(q)(S), on eta was Shown to be described satisfactorily by the empirical function, A eta(-alpha). It was shown for 9-methylanthracene in methylcyclohexane that the exponent, alpha, has a value of 0.68 +/- 0.06, which is independent of temperature and pressure but that the value of the constant of proportionality, A, is temperature dependent. The dependence of A on temperature was described empirically using the functions T-1.36 Or exp(-Delta E/RT). An alternative function for k(q)(S) is (T/eta)(beta), but this function failed to describe correctly the s dependence of k(q)(S) on eta and T. The fractional power dependence of k(q)(S) on eta suggests that the rate constant for the intrinsic fluorescence quenching reaction, k(act) is not significantly larger than the rate constant for diffusion. The values of k(q)(S) for the anthracene derivatives containing one or two electron-attracting substituents were smaller than that found for anthracene, decreased with Hammett’s sigma(p) parameter, and could not be described by the empirical function A eta(-alpha) With a constant value for alpha. A typical example is 9,10-dicyanoanthracene whose values for k(q)(S) exhibit a convex dependence on 1n eta. In accordance with this, the activation energy associated with k(q)(S) 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 T-1 states of the anthracene derivatives, k(q)(T) did not vary significantly, all being approximately 3 X 10(9) M(-1) s(-1) at 25 degrees C and 0.1 MPa. The logarithmic values of k(q)(T), when plotted as a function of In eta, displayed appreciable downward curvature far ail of the anthracene derivatives examined in this work.