The reduction reaction of a number of benzophenones with sodium borohydride in the solid state was studied, This type of organic solid-state reaction, avoiding a solvent, can be useful in chemical synthesis, The reaction was followed by in situ X-ray powder diffractometry and by H-1 NMR spectroscopy on subsequently taken aliquots. We found a strong dependence of the reaction rate on the water content of the mixture and on the type of benzophenone.