Interaction of impurities with the “visible defects” in hot implanted Cr doped semi-insulating (100) GaAs has been investigated. The defects studies were performed using transmission electron microscopy (TEM) and MeV He+ channeled Rutherford backscattering. The defects distribution was obtained by 90° cross-sectional TEM (XTEM). The atomic concentration profiles of Se, and carrier-concentration and mobility profiles were obtained by secondary ion mass spectrometry (SIMS) and Hall measurements in conjunction with chemical stopping, respectively. Comparison of defects, atomic and electrical profiles, showed the formation of secondary defects at and beyond the projected range (Rp), a significant amount of Se+ diffusion beyond Rp, and compensation of electrical carriers caused mainly by the point defects present in hot implanted GaAs.