Second-order rate constants (k(quin)) for the reactions of O-Y-substituted phenyl thionobenzoates (3a-3i) with quinuclidine have been measured spectrophotometrically. Comparison of k(quin) with the rate constants reported previously for the corresponding reactions with benzylamine (k(BzNH2)) has revealed that quinuclidine is less reactive than benzylamine toward 3a-3i although the former is 2.1 pK(a) units more basic than the latter. Steric hindrance exerted by quinuclidine has been suggested to be responsible for the decreased reactivity of the tertiary amine. The Bronsted-type plot for the reactions of 3a-3i with quinuclidine is linear with beta(1g) = -0.37. The Hammett plot correlated with sigma(o)(Y) constants exhibits many scattered points (R-2 = 0.982). In contrast, the Yukawa-Tsuno plot results in an excellent linear correlation (R-2 = 0.9992) with rho(Y) = 0.96 and r = 0.51, indicating that a negative charge develops partially on the O atom of the leaving group. Thus, the reactions of 3a-3i with quinuclidine have been concluded to proceed through a concerted mechanism in which expulsion of the leaving group is advanced only a little in the rate-determining transition state.