A divalent ion-selective electrode in combination with a single junction reference electrode was used to determine the stability constants of calcium, magnesium and zinc gallate. All measurements were carried out at 25[ddot]C, constant ionic strength of 0.1M and with the pH adjusted to pH8 using a Tris-HCl buffer. Direct potentiometric titration of the metal ion solution with solutions of gallic acid showed a break when the metal-ligand ratio was 2:1. Using the concentration of free metal ion determined by direct measurements, the experimental results indicated binuclear complex formation. Conditional stability constants were calculated. From the experimentally determined dissociation constants for gallic acid (pK14.4, pK28.6, pK311.2, pK412) the calculated stability constants for the metal gallate complexes gave the following log K values: Ca 11.45: Mg 10.70: Zn 11.38. The ion-selective electrode provides a Convenient and accurate method for the determination of stability constants in aqueous solutions.