Severe reactions are reported in the molten salt synthesis of niobates with the tungsten bronze structure. In the preparation of PbNb2O6 from PbO and Nb2O5, pure PbNb2O6 cannot be obtained using NaCl and KCl (Kimura et al., 1983b). When KCl is used as the molten salt, K ions extensively substitute for Pb ions and the product is a solid solution (1-x)PbNb2O6-xKPb2Nb5O15 with the tungsten bronze structure. The extent of the substitution, x, is determined by the heating conditions. When NaCl is used, the substitution reaction proceeds to form the (Na,Pb)NbO3 phase as well as (1-x)PbNb2O6-xNaPb2Nb5O15. The reaction PbO+2MCl→M2O+PbCl2 (M=Na, K) is responsible for this substitution.