The material balance serves as a major tool for the analysis of UCG (underground coal gasification) data; however, it must be used with considerable care. An oxygen balance, for example, can yield spurious and misleading predictions. Both field test data and laboratory experiments are used to show that a method currently used by Cena and previously by other investigators is so unstable that it is not a viable analytical tool. A more restricted form of the material balance is shown to produce good results.