The form of the solubility curves for VPE and LPE grown material are rationalised using a thermodynamic model which predicts that compensation of donors in VPE material is due to the donor-gallium vacancy complex Ge Ga V − ga whereas, in LPE material, the dominant acceptor making the material p type is the arsenic substituted state Ge − As. Mass action constants for the incorporation reactions for these entities and for the donor Ge + Ga are obtained. The compensation ratio in crystals grown from a congruent melt is predicted to be N D / N A ~ 5.9, and the transition to p type conductivity as one grows from Ga-rich solutions at progressively lower temperature is shown to occur at less than 100°C below the congruent melting point so that all LPE material should be p type. The superlinear behaviour of the total-germanium solubility curve at very high doping levels is postulated to be due to the formation of neutral Ge GaGe As pairs.