A graphical thermodynamic method is used to derive 6 "contamination-reaction rules." They relate the order of disappearance of phase assemblages to variations of temperature, pressure, and activities of mobile components when compositions change in a particular way. Application is based on data readily ascertained in field and laboratory. The rules provide one possible analytical basis for the subdivision and arrangement of the mineral facies of rocks. They also emphasize the need for caution in applying results of experimental study of "clean" compositional systems to petrogenetic interpretation.