We present results of optical degradation experiments on II-VI laser diodes. The illumination is done with a microfocus to allow the degradation of stacking-fault-free regions. A degradation is only observed if the excited region (in our case the waveguide) is doped with nitrogen. Parallel to the decrease of the quantum-well luminescence, we observe the increase of a deep luminescence signal from the waveguide region. Under reverse bias the degradation rate is decreased. Based on density functional calculations, this behaviour can be explained by a deplacement of the nitrogen from the substitutional to the interstitial site. The resulting complex of a selenium vacancy and the nitrogen interstitial is considerably more stable than the nitrogen acceptor.