The surface composition of binary mixtures of the protic ionic liquids ethylammonium nitrate and propylammonium nitrate has been investigated using surface tension measurements and the perfectly surface sensitive method metastable induced electron spectroscopy. Given that the latter technique is sensitive only to the outermost layer, it allows for the determination of the surface fraction occupied by a given species. The piecewise linear relationship between surface fraction and surface tension found in this study can be described by a phase separation within the surface layer.