Electrochemical scanning tunneling microscopy (STM) was used to study n type InP surfaces in an aqueous sulfuric acid solution. Well-defined InP(001), (111)A, and (111)B surfaces were prepared by chemical etching. in 1 M HCl. Cathodic polarization of the InP electrodes effectively protected the surfaces from oxidation. However, when the potential was made too negative, the decomposition of InP surfaces inhibited stable STM imaging. Atomically flat terrace-step structures were consistently observed on all surfaces in 0.05 M H2SO4 under suitable cathodic potentials. Steps observed on these surfaces corresponded to the monolayer steps with heights of 0.29 and 0.34 nm for the (001) and (111) surfaces, respectively. Atomically resolved STM images of InP electrodes were successfully obtained on the terraces for the first time. Ln situ STM revealed that all these surfaces possess a (1 x 1) structure with an interatomic distance of 0.42 nm. These results clearly demonstrate that the ideal InP(001), (111)A, and (111)B surfaces can be exposed in H2SO4 solution by the chemical etching method.