The application of structural models to some calcium borosilicate melts was made to develop simple expressions of activities as functions of composition. Their applicability was tested with freezing point depression data from phase diagrams. The activity values obtained were compared with those computed from the calorimetric heat of fusion. Data from published phase diagrams for the systems Ca2B2O5- Ca( BO2) 2, Ca3( BO3) 2- Ca2SiO4, Ca( BO2) 2- CaSiO3, and Ca2B2O5- CaSiO3 were used. Simple ionic models seemed applicable to melts where isolated ions may be expected; in melts containing mixed ions in chains and rings, the so-called "group" model may be useful. The authors, are respectively, Professor of Engineering, University of California, Los Angeles, and Professor of Inorganic Chemistry and Director, Institute of Silicate Science, Norwegian Institute of Technology, Trondheim, Norway.