The cathodic decomposition of the n-InP(100) plane and followed by anodic dissolution of deposited indium were investigated in relation to surface roughening. The cathodic decomposition proceeded in parallel with hydrogen evolution when the n-InP(100) plane was cathodically polarized in 1.0 M HCl at potentials lower than -0.6 V (vs. standard hydrogen electrode). Metallic indium was deposited as a result of the cathodic decomposition. and anodically dissolved at potentials higher than -0.5 V during anodic potential sweep after the cathodic decomposition. The reaction fraction of cathodic decomposition could be determined from the ratio of the charge for anodic dissolution of deposited indium to the total charge for cathodic decomposition and hydrogen evolution, and took a maximum value of x = 0.5 for cathodic polarization at -0.75 V for 50 s. The changes in surface morphology of InP were observed with an atomic force microscope. The roughness of the InP surface increased with repeating the cathodic decomposition and anodic dissolution of deposited indium. When the cyclic potential steps between -0.75 V for 30 s and -0.3 V for 30 s were performed, the mean value of surface roughness, R-a, increased with increasing cycle number up to 150 cycles and then attained a steady state of R-a = 25 nm.