The results of a study of the decomposition process at 1073-1123 K of the sulfates, nitrates, carbonates, and orthotellurates of alkaline and alkaline-earth metals in molten lithium, sodium, and potassium polyphosphates are compared. It is determined that the electrical conductivity increases, whereas the average molecular weight of the polyphosphates and the dynamical viscosity of the melts decrease with increasing contents of the modifiers in the starting mixtures and reaction time due to accumulation of the low-molecular-weight phosphates. The corrosion resistances of Armco iron and a steel in the modified polyphosphates are evaluated.