Spontaneous oscillations in current have been observed during anodization of InP. Oscillations were observed under three significantly different sets of conditions, potential sweep, constant potential, and constant current, for InP in an aqueous (NH4)(2)S electrolyte. Their frequency was found to increase with increasing anodic bias and values in the range of 0.1 to 1.25 Hz were recorded. Frequency was proportional to current density regardless of whether the measurements were carried out during a potential sweep or at constant potential. Thus, the charge passed per oscillation remained constant. A characteristic value of similar to0.3 C cm(-2) was measured under potential sweep conditions, and a similar value was obtained under constant potential conditions. More complex oscillatory behavior was observed under constant current conditions. The current in the oscillatory region corresponds to the continuous growth of a thick porous film on the electrode. (C) 2002 The Electrochemical Society.