AbstractConductometric titrations of mercury (II) perchlorate in presence of glycolic acid, hippuric acid, malonic acid and succinic acid were carried out according to JOB's method of continuous variation. A reaction mixture contained X ml of the acid and (20 – X) ml of mercury (II) perchlorate, the two components being of equal molarity. The presence of only one but well‐defined minimum in the plot of observed conductance versus X, for hippuric acid, oxalic acid, malonic acid and succinic acid showed that one complex of mercury (II) was formed in each case. The stoichiometry of the carboxylic acid complexes of mercury (II) was determined from the position of these minima and it was concluded that mercury (II) binds one hippurate, four oxalate the two malonate ions. The plot of observed conductance against X for glycolic acid, on the other hand, is characterized by the absence of any minimum and is in fact a straight line showing no interaction between mercury (II) and glycolate ion or an interaction too weak to be detectable by the presence method.Extending JOB's approach to conductometric data an attempt was made to derive quantitative expressions for the association constant of mercury (II) – carboxylic acid systems. The magnitude of association constants thus calculated showed that the carboxylic acids combined with mercury (II) with a decrease in free energy of about 5–6 kcalories/mole and that malonate is more effective than hippurate in binding mercu