We report the first evaluation of interface states present in GaAs p-n junctions regrown on a SiO2 masked substrate by selective molecular beam epitaxy. A constant-capacitance deep-level transient spectroscopy (CC-DLTS) method is extended to the regrown p-n junction case, and a technique to distinguish the DLTS signal emerging from bulk and interface states is employed. The extracted interface state density is in good agreement with predictions based on the unified disorder-induced gap state model.