A theory of condensed-phase transitions during vaporization processes, i.e. a theory of triple points, was tested by mass-spectrometric observation of the effusing vapor of gallium sesquisulfide, Ga2S3(c), as the condensed phase underwent a phase transition both in the increasing-temperature direction and in the decreasing-temperature direction through 25 cycles. Theoretical prediction of the trajectory of partial-pressure ratios in pressure, temperature, composition space through a temperature-driven phase transition was tested by measuring ionic intensity ratios. The vapor species, principally Ga2S(g) and S-2(g), had a ratio of partial pressures independent of temperature while either the low-temperature or the high-temperature condensed phase was effusing congruently, but the ratio showed strong predictable dependence on temperature and composition during a condensed-phase transition, in line with the theory. This work is the first direct test of these theoretical predictions.