Second-order rate constants (kN) have been measured spectrophotometrically for reactions of Y-substituted phenyl benzoates (1a-h) with azide ion (N3-) in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C. The Brønsted-type plot for the azidolysis exhibits a downward curvature, i.e., the slope (βlg) changes from -0.97 to -0.20 as the basicity of the leaving group decreases. The pKao (defined as the pKa at the center of the Brønsted curvature) is 4.8, which is practically identical to the pK a of the conjugate acid of N3- ion (4.73). Hammett plots correlated with σo and σ- constants exhibit highly scattered points for the azidolysis. On the contrary, the corresponding Yukawa-Tsuno plot results in an excellent linear correlation with ρ = 2.45 and r = 0.40, indicating that the leaving group departs in the rate-determining step. The curved Brønsted-type plot has been interpreted as a change in the rate-determining step in a stepwise mechanism. The microscopic rate constants (k1 and k2/k-1 ratio) have been calculated for the azidolysis and found to be consistent with the proposed mechanism.