A general system of equations describing the dissolution of polydisperse solids having a porous particulate structure is set up and applied to study the hydrolysis and subsequent dissolution of silicon nitride powder in aqueous suspensions. The latter process has been found to follow a simple kinetic equation, c(t) = c(s)[1 - exp(-kt)], with the dissolution rate constant k = 2.3 x 10(-6) s(-1) (or 2.5 x 10(-11) mol m(-2) s(-1)) and the saturation concentration c(s) = 3.2 mmol dm(-3) at room temperature and pH 9. The role of diffusion is unimportant, so the concentration of soluble species in pore water changes almost simultaneously with their concentration in the surrounding solution.