The effect of 300‐keV proton bombardment and subsequent annealing on the optical absorption and electrical resistivity of bulkp‐type (p=2×1018and 1.4×1019cm−3) GaAs has been studied. Proton doses were in the range 1013−1017cm−2. It is found that bombardment‐induced optical absorption increases monotonically, but sublinearly, with proton dose. The shape of the optical transmission spectrum indicates that bombardment creates a distribution of energy levels extending into the forbidden gap. Activation energies for annealing of the optical absorption have been determined from isothermal annealing data and range from ∼1.5 to ∼3.4 eV, which indicates that at least two kinds of defects are involved. Current‐voltage measurements show that the average electrical resistivity of the bombarded layers goes through a maximum at ∼2.5×105&OHgr; cm at a proton dose of ∼3×1015cm−2. It is shown that annealing can eliminate the bombardment‐induced optical absorption while still retaining a high electrical resistivity. The optimum annealing time and temperature is a function of the proton dose. From these results a set of useful conditions for the proton‐bombardment fabrication of stripe‐geometry GaAs lasers is determined. Many qualitative similarities exist between the results presented here and those obtained in GaP, which are presented in the following paper.