Homogeneous (unseeded) nucleation of Mg phosphate from modified Ca-free seawater solutions was investigated at 20 degreesC and pH of 8. Precipitated solid phase was characterized using chemical analysis, X-ray diffraction, scanning electron microscopy, and IR-spectroscopy. Effect of aqueous phosphate and ammonia concentrations and the intensity of stirring on the induction period time (tau) of Mg phosphates nucleation were studied. A linear relationship between logarithm of bobbierite (Mg-3(PO4)(2) . 8H(2)O) saturation index and log tau was established with a slope close to 2, Aqueous NH4+ (up to 0.002 M) has no effect on the nucleation kinetics and does not incorporate in the precipitated solid phase. Stirring of solution has a dramatic effect on nucleation kinetics: the induction period decreases by a factor of 100-10000 in unstirred solutions compared to stirred ones. The relative diffusion/chemical reaction control mechanism of Mg phosphates precipitation from supersaturated solutions is discussed. It is shown that spontaneous inorganic precipitation of Mg-(ammonium) phosphates (struvite and bobbierite) in modern marine environment is impossible because of very sluggish kinetics.