InP epi‐layers heavily doped with Se exhibit very high carrier concentrations (N≳1020 cm−3) together with significant increases of the optical band‐gap energy. We show that in these epi‐layers (grown by liquid phase epitaxy) the Se concentration XSe varies with the depth. With increasing Se concentration, the optical band gap increases from 1.34 to ∼1.9 eV, where it remains fixed for Se concentrations larger than 1.5 at.%. This increase is very rapid at first owing to the filling of the low mass conduction band. Beyond XSe = 0.1 at.%, we enter the nonarapbolic bands regime and the electrons begin to overflow into the indirect minima. Because of the large density of states available, further increases in free carriers no longer affect the optical gap, which remains close to the indirect gap value (∼1.9 eV).