Ultraviolet photoelectron yield spectroscopy (UPYS) was applied to the characterization of Langmuir layers of the amphiphilic Ru complex at the air/water interface in ambient conditions. The UPY spectra were successfully obtained for Langmuir layers of the complex in the surface concentration range of 0.14-2.1 molecule nm(-2), and the photoelectron emission threshold energy Et and photoelectron emission efficiency, (x, were determined for the respective surface concentrations. The Et value was constant at lower surface concentration and decreased as the surface concentration of the complex was increased. The decrease in the Et value is considered to be due to the decrease in the distance between the Ru complexes in the two-dimensional gas phase and the following aggregation or crystallization considering the polarization effect on the photoionization hole state. The dependence of the a value upon the surface concentration of the complex also indicates the change in structure of the Langmuir layer of the Ru complex on the water surface. It is concluded that UPYS is applicable to the in situ characterization of Langmuir layers of photosensitive molecules at the air/water interface.