The grain size variation in "unseeded" Si3N4 powders, prepared by high-pressure carbothermal nitridation of SiO2 (in stoichiometric 1:2 proportions with C), has been studied by means of scanning electron microscopy (SEM) and "Sedigraph" measurements. The size is a function of process parameters, of which the reactant surface area was found to be the most important. Specifically, with an SiO2 area A(SiO2) approximate to 50 m(2) g(-1) in the reaction mixture, the resulting mean Si3N4 particle diameter, d(Si3N4), is very sensitive to the carbon surface area, A(C), such that the minimum d(Si3N4) approximate to 1 mu m was obtained with A(C) = 30m(2) g(-1) and the maximum d(Si3N4) approximate to 7 mu m with A(C) = 115 m(2) g(-1). Using mixtures with A(SiO2) = 50 m(2) g(-1) and A(C) = 115 m(2) g(-1), a slight dependence of d(Si3N4) on the furnace heating rate was a Iso observed; larger grains (approximate to 7 mu m) were obtained with 20 deg min(-1) than with 2 deg min(-1) (approximate to 5 mu m). The grain size was found to be virtually independent of nitrogen pressure (in the range 0.3-6.5 M Pa), annealing temperature (1470-1830 degrees C) and gas flow rate (2-20 l((stp)) min(-1)).