A comparison has been made between rapid thermal annealing (RTA) and furnace annealing (FA) of implanted GaAs. Hall measurements showed consistently higher electrical activation of n-type (Si-5E12 cm −2) implants with FA and higher electrical activation of p-type (Be-1E14 cm −2) implants with RTA. Photoluminescence (PL) revealed that for RTA temperatures above 950°C some of the Si was going onto As, acceptor sites, thus reducing the net donor concentration. PL for RTA below 950°C showed signs of incomplete recrystallization. By a technique of “dual implantation” of As and Si into GaAs we were able to force the Si onto Ga-sites resulting in a higher donor concentration after RTA.