Activation of the Zn acceptors in AlGaInP epitaxial layers grown on misoriented substrates by metal organic chemical vapour deposition (MOCVD) has been successfully improved using post-heat treatment. The p-type carrier concentration and electrical activity on the (Al/sub 0.65/Ga/sub 0.35/)/sub 0.5/In/sub 0.5/P layers were 1.8*10/sup 18/ cm/sup -3/ and 0.7, respectively. Activation of the Zn acceptors was found to be attributable to the hydrogen in the AlGaInP layer.<<ETX>>