Thermostable bismuth vanadate/molybdate pigments are prepared by precipitating onto a pigment, obtained by calcination, of the formulaBi VO.sub.4 .cndot.Bi.sub.2 Mo O.sub.6where n is 0.2-2.5, in aqueous suspension at 70.degree.-100.degree. C. within the pH range from 4 to 9.5 a first SiO.sub.2 -containing layer using an alkali metal silicate solution of a magnesium hexafluorosilicate solution, maintaining the suspension at 70.degree.-100.degree. C. for not less than 1 hour, producing a thick coating on the precoated pigment by adding further alkali metal silicate solution, magnesium hexafluorosilicate solution or a mixture of the two within the pH range from 4 to 9.5, and finally depositing wax on the SiO.sub.2 -containing layer by means of a waxing emulsion/dispersion, the products showing, compared with the starting pigment, an appreciably improved thermal stability in plastics.