Complex formation reactions between In(III) and 2-mercaptopropanoic acid (H2A) have been studied in 3.0 M NaClO4 at 25°C, by measuring the EMF of the glass and In(Hg) electrodes as a function of the acidity of the solution. At -log|H+ > 5.5 the formation of a precipitate is observed. The EMF data obtained in the range 1 < -log|H+| < 5.5, treated by the general minimizing program LETAGROP, can be explained by assuming the formation of the species InA+, InA(OH) and InA(OH)2-. A reaction equation is proposed and the formation constants for these species are given. © 1988.