The magnetocapacitance characteristics of an epitaxial p-n heterojunction between magnetic InMnAs and InAs are investigated. A large positive magnetocapacitance is observed at room temperature, which increases with reverse bias. For high reverse bias, the magnetocapacitance is linearly dependent on magnetic field. From capacitance-voltage measurements, the junction built-in voltage was determined and was observed to increase with magnetic field. The magnetocapacitance measurements support a model for a magnetic semiconductor heterojunction where spin-split polarized valence and conduction bands form due to the giant Zeeman effect. (C) 2008 American Vacuum Society.