Simultaneous doping: (codoping) of n- and p-type impurities in GaAs is investigated in both uniformly and delta -doped structures using Si and Be as n- and p-type dopants. In highly Si-doped GaAs layers, the electrical activation ratio of Si-donors is much less than unity because of the formation of compensating centers such as antisite Si (Si-AS), Si-clusters and Ga vacancies (V-Ga) Although no enhancement is observed in the electrical activation, it is found that most of these compensating centers disappear as a result of codoping regardless of the doping structures. The integrated photoluminescence intensity in the codoped GaAs layers is much higher than that in the solely Si-doped samples. indicating the reduction of nonradiative recombination centers. The uniformly doped samples show very large spectrum widths, while the delta -doped samples keep narrow line with even for increased Be concentration.