One of the fascinating subjects in the researches in soft interfaces including liquid/liquid interfaces is heterogeneity of interfacial films, because it is of importance to understand the domain formation such as “raft” in biological membranes. In this study, the domain formations in the adsorbed films at the hexane/water interface were examined by interfacial tensiometry and X-ray reflection (XR). The adsorbed F8H2OH film shows an expanded - condensed phase transition. The expanded state is heterogeneous film in which the condensed domain coexists with low density gaseous region. The domain coverage strongly depends on the line tension acting on the domain boundary. In the mixed F8H2OH - F6H6OH system, the expanded - condensed phase transition becomes obscure with increasing the fraction of F6H6OH in the adsorbed film. XR analysis indicated that the coverage of condensed F8H2OH domain increases and the size of the domain decreases by mixing of F6H6OH with F8H2OH in the adsorbed film. This is probably due to the reduction of domain line tension by preferential adsorption of F6H6OH at the F8H2OH domain boundary. Furthermore, the domain formation in the mixed C14PC - Cholesterol film was examined by coupling the 2D phase diagram and XR data analysis.