A process for the preparation of a stable dispersion of solid particles, in an aqueous medium comprising combining (a) a first solution comprising a substantially water-insoluble substance which is a pyrazine compound of Formula I, a water-miscible organic solvent and an inhibitor with (b) an aqueous phase comprising water and optionally a stabilizer, thereby precipitating solid particles comprising the inhibitor and the substantially water-insoluble substance; and optionally removing the water-miscible organic solvent; wherein the inhibitor is a non-polymeric hydrophobic organic compound as defined in the description. ##STR00001## Also claimed are stable dispersions prepared by the process, solid particles prepared by the process and use of such particles.