Current-voltage and impedance spectroscopic measurements were made at the p-InP < 100 >/H2SO4 contact. Small reverse saturation current densities (ca. -1 nA cm(-2)) at low doped samples allowed measurement of the space-charge capacity C-sc over more than seven frequency decades (20 mHz-300 kHz). An extremely low and potential-independent frequency dispersion of C-sc of 0.4% per frequency decade was obtained (20 Hz-20 kHz). Results obtained from capacitance data for the semiconductor-surface energy barrier height phi(B) with and without platinum coating and for different doping densities are compared with data from photopotentials. Larger variations of phi(B) (0.7-1.0 eV) than those typically achieved at solid state p-InP-metal contacts are verified. Flatband potentials and photopotentials are used to estimate the band-bending under illumination which, at bare and Pt-coated p-InP-electrolyte contacts, turns out to be 70-240 mV lower than at ideal Schottky contacts.