GaSb layers grown by LPE in a sapphire boat exhibit higher free hole concentrations than layers grown at the same temperature but in a graphite boat. In both cases, as the growth temperature is lowered, the free hole concentration diminishes and eventually the layers' conductivity change to N type. The transition temperature is lower for the samples grown in the sapphire boat. This behavior is explained assuming that the residual impurities coming from the graphite boat are mainly donors, making the concentration of the native accepters less compensated in GaSb grown in the sapphire boat. The free carrier concentrations and their mobility were determined by infrared reflectivity. Evidence of the compensation effect was obtained from the photoluminescence spectra of the samples.