Understanding the stability of metal halide perovskite (MHP) surfaces is of considerable interest for the development of devices based on these materials. We present here a vacuum-based study of the surface potential and response to illumination of two different types of perovskite films, methylammonium lead bromide (MAPbBr3) and the 2D Ruddlesden-Popper phase butylammonium lead iodide (BA2PbI4, n = 1), using Kelvin probe-based contact potential difference and surface photovoltage measurements. We show that supraband gap light irradiation can induce the loss of halide species, which adsorb on the Kelvin probe tip, inducing quasi-irreversible changes of the MHP surface and tip work functions. If undetected, this can lead to misinterpretations of the MHP surface potential. Our results illustrate the effectiveness of the Kelvin probe-based technique in providing complementary information on the energetics of perovskite surfaces and the necessity to monitor the work function of the probe to avoid erroneous conclusions when working on these materials.