When a binary system A(x)B((1-x)) vaporizes congruently, the composition of the equilibrium vapor is such that the atom fractions of the elements entering the vapor are the same as those in the condensed phase. With temperature, the congruently vaporizing composition may vary continuously or may change discontinuously when a condensed-phase transition occurs. Such variations in the congruently vaporizing composition can be detected by monitoring the composition of the vapor phase. Condensed-phase transitions can cause anomalous vaporization behavior such as inverse dependence of vapor pressure on temperature or even increase in vapor pressure at constant temperature, as well as drastic changes in the composition of the vapor with temperature. However, such occurrences might be missed or dismissed as spurious if consideration be given only to data which follow Clausius-Clapeyron straight lines. This paper illustrates these points with results reported in the literature for three binary systems Ga/S, Ga/Se, and Mn/Tz. Certain effects associated with phase transitions in these systems are discussed in detail. Other systems in which anomalous effects might occur are discussed in brief. The potential of high-temperature mass spectrometry for the study of such effects is highlighted.