The use of bis(methylcyclopentadienyl)magnesium (MCp2Mg) as ap-dopant source for MOCVD-grown InP has been investigated. The Mg incorporation was nonlinear. The relationship between the H2 flow through the MCp2Mg bubbler and the Mg concentra-tion in the epilayers suggested that when [Mg] 1019 cms-3 the layer morphology deteriorated and stacking faults were observed by TEM, at a density greater than 109 cms−2. Significant diffusion of Mg into the substrates during the growth was observed, with diffusion depths up to 0.1 µm at a concentration of 1019 cms−3 in S-doped, and up to 32 µm at 1017 cms-3 in Fe-doped substrates. These concentrations correspond to the S and Fe doping level in those substrates, and the results are explained in terms of the formation of a complex between the S or Fe dopants and the diffusing Mg, which immobilizes the latter species. At [Mg] >1018 cms−3, the net hole concentration, measured by means of electrochemical C-V pro-filing, decreases with increasing [Mg], indicating significant self compensation. Com-pensation at high [Mg] was also suggested by the effect of excitation power density on the peak shift of the donor to acceptor transition observed during photoluminescence measurements at 7 K.