BACKGROUND: There has been considerable controversy over the effect of seed crystal additions on crystallization of magnesium ammonium phosphate hexahydrates (MAP). In this study, batch experiments were carried out to verify this effect using synthetic solutions. RESULTS: Addition of seed crystals could increase the rate of MAP crystallization and significantly shorten the reaction crystallization time, especially for solutions with a lower supersaturation ratio (Omega). Crystallization time of MAP with and without seed addition can be expressed by two different formulas, in which the crystallization with feed addition followed an exponential function, and the secondary formula fitted a two-order power function. When 0.422 g L(-1) of seed crystals was added the equilibration time for crystallization calculated by the formula was reduced by 75min for low supersaturation ratio solution and 26min for medium supersaturation ratio solution. CONCLUSION: To increase the MAP reaction rate and to shorten induction and equilibration times, seed crystal addition should be practiced (C) 2011 Society of Chemical Industry