MgNH(4)PO(4)center dot 6H(2)O (struvite) was produced continuously in aq. soln. by reacting PO(4)(3-) ions (1.0 % by mass with P ions (0.0025-0.025 % by mass) or SiF(6)(-) ions (0.0125-0.050 % by mass) at 298 K and pH 9 for 900 s (residence time). Properly shaped struvite crystals (mean size 30-47 mu m) were formed. Higher concns. of the ions favored prodn. of crystals of larger sizes (even by 35%) at confined nucleation rate and at simultaneously higher linear growth rate (ca. 2-10(-8) m/s). Excess of Mg(2+) ions in the feed related to PO(4)(3-) and NH(4)(+) ion contents (molar ratio 1.2:1:1) influenced the process results advantageously. Mean product crystal size increased by 3 to 13%, while crystal linear growth rate increased by ca. 10%.