In this work we studied some fundamental concepts of n-InP interfacial properties based on acid-base equilibrium in the dark and also after the surface illumination at open circuit voltage (Voc). The variation of the flatband potential on undoped n-InP semiconductor was investigated according to aqueous pH. This study was complemented by the effect of the illumination at Voc conditions through photopotential measurements. The impact of the surface lighting on acid-base equilibrium was analyzed by in-situ interfacial capacitance measurements in the dark and by ex-situ X-ray photoelectron spectrometry. (C) The Author(s) 2018. Published by ECS.