Theoretical expressions derived previously for molecular size distributions in multichain polymers are applied to binary silicate melts. The treatment is an extension of a previous approach which was limited to the consideration of linear chains. When all configurations of the chain molecules are taken into consideration, the predicted variation of thermodynamic activity with composition agrees with experiment for all binary systems for which data are available. The effect of allowing for all chain configurations is largely to improve the fit between theory and experiment at high silica contents. Calculated ionic distributions for the system 'FeO'–SiO2 do not differ markedly from those previously reported. The mean chain length is unaffected. The results support previous views that principles of polymer chemistry can be applied usefully to silicate melts and glasses.