Original data on ultrasonically determined elastic moduli of the vitreous systems Co-P-O and H-P-O have been used as a phase separation test for these glasses. For the Co-P-O system whose compositions ranged from 5 to 60 mol% CoO, no evidence for phase separation in any part of the composition range 0 to 42 mol% CoO could be found. The possibility of a narrow miscibility gap lying somewhere within the remaining composition range cannot be ruled out, though electron microscopy gave negative results. In the case of the H-P-O system whose compositions ranged from 0 to 50 mol % H2O, the possibility of a miscibility gap somewhere within this range was indicated by the elastic moduli data. It would be well worthwhile to perform other kinds of phase separation tests on this system, though this will be difficult because of the rapidity with which water is absorbed. A qualitative theoretical interpretation of the results is attempted, from which it is concluded that the phase equilibrium of these phosphate glass sytems is determined by the vibrational energy and entropy at the melting temperature rather than the zero-point internal energy.