Estimation of the steam fraction of injected water in vapor-dominated geothermal fields is a research priority in the design of the injection scheme. Tracer tests using volatile tracers can be a practical method for obtaining this estimate. Low molecular-weight alcohols were selected as two-phase tracers at the Matsukawa field because their volatility is much closer to that of water than vapor-phase tracers. This property allows the two-phase tracers to approximate the boiling behavior of injected water in a reservoir and to estimate both the flow path and the steam fraction. Tracer tests were performed in three injection wells at Matsukawa. Aqueous solutions of two kinds of alcohols were used in three injection wells because the use of concentration ratio of the two tracers eliminates the effect of tracer dilution in the steam fraction calculations. The tracer concentration ratio can be expressed as a function of the steam fraction and the vapor-liquid distribution coefficient of the tracers. Single stage boiling and continuous boiling models were used to represent closed and open system boiling, respectively. The results of the tests implied the occurrence of continuous boiling for the Matsukawa injectate and provided a range of steam fractions. Differences in the calculated steam fractions were probably due to varying degrees of heat recovery by the injected water, which is influenced by the temperature distribution in the reservoir.