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) = cs[1-exp(-kt)], with the dissolution rate constant k = 2.3E-06 1/s and the saturation concentration, cs = 3.2 mmol/l at room temperature and pH9. The role of diffusion is unimportant, so that the concentration of soluble species in pore water changes almost simultaneously with their concentration in the surrounding solution.