The formation and the composition of isopolymolybdates have been studied by measuring the magnetic susceptibilities of solutions of normal sodium moIybdate in aqueous nitric acid of progressively increasing concentration. The susceptibility—pn curve shows strong deviation from the mixture law and is characterised by having four well-defined inflexion points or breaks, corresponding to the addition of about 4, 7, 9 and 10.5 H-ions for every 6MoO4-ions. formation of di-, para-, meta- and octo-molybdates with compositions, as advocated by Jander and co-workers, is thereby confirmed.