The esterification of propionic acid with n-butanol at 126-133 degrees C in the presence of catalytic quantities of novel aluminophosphate molecular sieve AP-9 gives the corresponding esterified product in a high yield. The effects of the amount of catalyst used, reaction time, the molar ratio of alcohol to acid, the amount of cyclohexane used, reaction temperature, the kinds of catalyst, and the methods of preparing the catalysts, on the synthesis of butyl propionate have been investigated. The yield of butyl propionate obtained was 85.3% when the reaction conditions employed were as follows: catalyst: propionic acid: n-butanol: cyclohexane = 1.0 g: 1 mol: 1.2 mol: 5.0 ml, at 126-133 degrees C for 5 h. The aluminophosphate molecular sieve AP-9 can be recovered and reused to give almost the same yield as that given by a fresh aluminophosphate molecular sieve.