The rate of the Mannich reaction between 3-pentadecylphenol, di-n-propylamine and 40% aqueous formaldehyde in aqueous ethanol under basic conditions has been studied by an HPLC analytical technique following preliminary experiments with dimethylamine, diethylamine and morpholine. The effect of excess amine and excess formaldehyde, different temperatures and the influence of pH were investigated. Kinetic expressions involving the concentrations of the phenol, amine and formaldehyde have been obtained on the assumption of the participation of either (a) the aminal (methylene-bis-amine) or (b) the aminol (methylolamine) for five cases covering the various molar proportions of reactant used experimentally. Initially, to test the equations for reactions effected at constant water concentration, theoretical values of t (time) were obtained as the initial concentration (a) of the phenol was varied by increments by a/20 using trial values for the composite rate constant. In this manner the best visual fit between the experimental points and the theoretical curves was obtained. The rate constant corresponding to such a curve was subsequently refined using a least squares procedure to yield the value which minimised the sum of the residuals between the experimental and theoretical values. In all cases the sum of the residuals was smaller when the kinetic expression derived assuming participation of the aminal was employed. The latter assumption also led to a smaller dispersion in the values obtained for the composite reaction constant from the different experiments.