In spite of the generally good biocompatibility of TiNi revealed in numerous in vivo studies, medical applications of these shape memory alloys as implants are hindered due to the lack of knowledge on the nature of their biocompatibility. Better material characterization is necessary for understanding the chemical and physical properties which determine biocompatibility. The purpose of the present investigation is to use angle-resolved x-ray photoelectron spectroscopy to identify the differences in surface chemical state and composition between the layers formed on the surface of TiNi alloys after mechanical polishing, chemical etching, autoclaving in water and steam, and exposure to H2O2 or air. The surface chemistry drastically depends on the preparation method; the Ni surface concentration can be varied in the range 0-30 at. %. In natural conditions (air, water, and steam) TiNi surfaces revealed a tendency to be covered by TixNiyO2x oxides (where x varies in the range of 7-20 and y varies in the range of 0-5 depending on preparation procedure) with a small amount of Ni in metallic state (similar to 0.5 at. %). This finding is considered to be the reason for the good biocompatibility of TiNi.