Values of glass transition temperature (T g ) for several samples of sodium copolyphosphates were determined by DSC measurements. Their dependence on a number of factors has been considered. Contrary to the observations ofEisenberg et al.T g does not depend entirely on the ratio of the cation charge, $$\bar q$$ , and the inter-nuclear distance between the cation and the chain anion,a. For a particular kind of copolyphosphate it depends remarkably on the conditions of formation viz. the temperature of the melt quenched to form the glass and the time duration for which the melt remained as such before being quenched. When the temperature of the melt is kept constant and the time duration varied,T g registers an increase with increase in time. Keeping the time constant and raising the temperature once again results in an increase in the value ofT g . The values ofT g are also found to depend on the size and the atomic weight of the cation, other than sodium;T g increases with its size and its atomic weight. With a few exceptionsT g is also found to increase with increase in the value of the number-average molecular weight.