From the precipitation borderline in the pM′-pH diagram, determined experimentally under CO2-free conditions, the stability constants of the mononuclear and polynuclear species of samarium hydroxide have been established. The values found are log*β1 = -7.5, log*β2 = -15.0, log*β3 = -22.7, logβ4, 3 = -19.5 and log*Ks0 = 17.5. They refer to fresh precipitates, prepared at room temperature in sodium perchlorate medium with an ionic strength of 1. © 1979.