Results on the activation characteristics of Be, Be+P, Zn and Zn+P implants In GaAs have been presented. It is found that the in-diffusion of zinc and the out-diffusion of beryllium atoms has been prevented through the co-implantation of phosphorus. High hole concentrations of the order of 6x1019/cm3 have been obtained for the Zn+P Implanted samples. It has also been demonstrated for the first time that by performing the co-implantation of phosphorus and rapid thermal annealing, an effective way of preventing the out-diffusion of beryllium at annealing temperatures up to 950°C for 10s has been achieved. The Influence of heat treatment on the rapid thermally annealed, ion-implanted samples has also been investigated. It is found that the temperature at which the electrical properties change significantly is at 600°c for the Be, Mg and Zn Implanted samples and at 700°C for the selenium implanted samples respectively. These changes in the sheet carrier concentration are observed to be reversible for the selenium and zinc implanted samples after further annealing at high temperatures. This phenomenon is not observed for the beryllium and magnesium Implanted samples. A value In the range of 1.5-2.5eV has been identified as the energy which characterises the rate dependent step for the reversible processes for the zinc and selenium implanted samples. This energy value is thought to correspond to the diffusion of gallium atom/vacancy. Similarly, an energy of about 1eV has been identified for the beryllium Implanted samples. It is concluded that the decrease in the sheet hole concentration for the beryllium implanted samples is due to the out-diffusion of the beryllium atoms. The value of 1ev is thought to be the energy which characterises the diffusion of beryllium.