The procedure based on the polymer theory of oxide melt structure has been proposed to calculate the ion composition and activities of components in CaO-Al2O3 melts. Different coordination of Al in the form of Al3+, AlOPLU and complex ions AlxOyz- is taken into account. Estimated data of the Al2O3 and CaO ion compositions and activities at 1773 K for 5 structural models. For one model, estimated activities are in close agreement with experimental values. Ion compositions for CaO-Al2O3 melts, obtained by the model equations, are listed. It is shown that higher mole fraction of CaO in the melt markedly decreases the Al3+ content. Most alumo-oxygen anions have the form of Al45- surrounded by Ca2+ cations.