Measurements of the induction period for gypsum nucleation, when citric acid is added in solution as an additive to retard calcium sulfate nucleation, are reported in the present paper. The supersaturation ratio was varied in the interval 4-5; the concentration of citric acid was varied from 0.01 to 0.30 g/L; and, finally, different temperature levels were explored in the range 15-45 degreesC. The induction period values were compared to those previously obtained by using the same experimental technique in the absence of foreign ions in the mother liquor, showing that the citric acid has a strong retarding effect toward gypsum nucleation. Eventually, several values for the interfacial tension and activation energy have been estimated as a function of citric acid concentration and of temperature.