The phenomenon of formation of mixed crystals during microbial conversions involving water-insoluble substrate and product was examined through the investigation of two biotransformations: (a) dehydrogenation of hydrocortisone by Arthrobacter simplex and (b) oxidation of cholesterol by Rhodococcus erythropolis. Evidence for the intrinsic ability of hydrocortisone and prednisolone molecules to form mixed crystals from both artificial solutions and microbial conversion systems was provided by x-ray powder diffractometry and differential scanning calorimetry. Mechanical agitation was shown to further promote the bioconversion of mixed crystals from an initial value of 83% prednisolone to an ultimate value of 95%. The solids in the bioconversion system of cholesterol were shown by their x-ray diffractions to be a mixture of single substrate and product crystals. A scheme of the various crystal structures likely to be formed in bioconversions—mixed single crystals, mixed crystals, and coated crystals—was presented. A solubility-based classification of the sterols and steroids with regard to their likelihood to form mixed crystals in bioconversion systems was suggested.