Interface morphology, phase composition, and elemental diffusion of Pt/Ti/Ge/Pd ohmic contacts to both n+- and p+-GaAs have been investigated as a function of annealing temperature. Structural and chemical results were correlated with specific contact resistances (rho(c)) measured for each thermal treatment in an attempt to determine how the indiffusion of contact metal and the out-diffusion of Ga and As affect specific contact resistance, and how effective Ti is as a diffusion barrier. Annealing at 450-degrees-C yielded the lowest rho(c), approximately 6.4 X 10(-7) OMEGA CM2 . The interface was observed to be smooth and abrupt, and there was little As out-diffusion and Pt indiffusion. Two interface phases were detected; a primary phase, PdGe, and a secondary, Ga rich Pd-Ga-As ternary phase. The presence of this ternary phase is believed to be critical to contact formation on n-type GaAs. The Ti and Pt layers remained pristine. Annealing at 550-degrees-C resulted in a slightly higher p(c), approximately 2.1 X 10(-6) OMEGA Cm2. There was significant elemental diffusion within the contact metal and minor diffusion into the GaAs substrate. The interface possessed limited areas of spiking with uniform composition. Annealing at 600-degrees-C proved to have a detrimental effect on the rho(c), approximately 10(-4) OMEGA CM2. This electrical degradation was accompanied by strong chemical intermixing between the contact and substrate, resulting in a continuous nonplanar interface with deep multiphase protrusions, and the Ti layer no longer was effective as a diffusion barrier.