The alloy composition of a Cd1-xZnxTe(111) sample and its spatial homogeneity have been determined by surface photovoltage spectroscopy (SPS) and compared to conventional energy dispersive x-ray spectroscopy measurements. Experimental improvements of the former technique yield a contactless, surface sensitive, and highly accurate spectral resolution of the band gap (error<4meV) and consequently of the Zn concentration (error<0.6% in comparison with the latter technique). In addition, SPS is capable of determining the face and type of the Cd1-xZnxTe as well as identifying gap states at its surface. The electronic structure has been investigated in comparison with n-CdTe(111), before and after various surface chemical treatments. An acceptor surface state has been observed at 1.21 eV below the conduction band edge and attributed to TeO2. A donor surface state (with a lower concentration relative to the corresponding state in CdTe) associated with Cd atom displacement has been found at 1.42 eV above the valence band maximum. A chemically induced surface state at 0.72 eV below the conduction band edge may be due to Zn vacancies, as supported by x-ray photoelectron spectroscopy measurements.