The kinetics of base-catalyzed hydrolysis of 4-nitrophenyl acetate was studied in water and water-ethanol mixtures under ultrasound (21.1 kHz) using a probe equipped with a quartz horn and, by comparison, without sonication. The ethanol concentration was varied from 0 to 50 wt % and pH from 7.5 to 9. The increase of reaction rate was inversely proportional to the strength of substrate interactions with the solvent structure. It was found that the effect of ultrasound could be related to the perturbation of solute-solvent interactions. This conclusion seems to apply to both acid-catalyzed and base-catalyzed hydrolysis reactions. Parallel experiments with a titanium horn revealed the catalytic action of undetermined chemical species formed from dispersed titanium metal.