For tin-tungsten bronzes SnxWO3 prepared both under mild, aqueous conditions and by a high temperature solid state reaction, the degree of hydration, the amount of X-ray amorphous byproducts and the value x have been determined quantitatively by thermogravimetric reduction in hydrogen. This is clearly more precise than the oxidation in oxygen, as the latter is not complete at 1050-degrees-C and is accompanied only by a much smaller change in mass.