This paper describes a simultaneous-coupling azo dye method for the measurement of esterase activity using the histochemical substrate, alpha-naphthyl acetate. By the choice of two diazonium salts with optimal coupling characteristics, the reaction can be carried out at any pH between 3.0 and 9.5. The azo dye is maintained in solution for spectrophotometric measurements with bovine serum albumin. The simultaneous-coupling method is compared with an assay based on the direct measurement of released alpha-naphthol by its ultra-violet absorbance in a pH study of hog liver esterase. There is good agreement between the data obtained by both methods.