Current–voltage (I–V), capacitance–voltage (C–V), and photovoltage (PV) data are obtained for Ru, Rh, Pd, Os, Ir, and Pt contacts deposited on chemically cleaned 〈100〉 n‐GaAs wafers. By explicitly evaluating the relevant equations using realistic values of all parameters, we show that the currents obtained for small voltages are far too large to be explained by thermionic or recombination effects alone but are indicative of barrier height variations as proposed by Freeouf e t a l. We used the fact that the thermionic emission component is always present to reconcile the difference between apparent barrier heights determined by I–V and C–Vmeasurements to obtain a consistent set of values for all materials. Upon exposure of the Ru and Ir contacts to atmospheric‐pressure hydrogen, the barrier heights are reduced by 160 and 70 mV, respectively, and the ideality factors by 0.3 and 0.3, showing that hydrogen simultaneously lowers the barrier height and makes it more uniform over the contact for these junctions. The introduction of hydrogen into the bulk of these hydrogen‐alloying metals has no effect on the transport properties of the Pt, Rh, and Pd junctions despite a reported work function change of nearly 1 eV for Pt, over 0.5 eV for Rh, and a phase change to a metallic hydride for Pd. The change in optical properties with ambient in the Drude region of Ru shows that the bulk Fermi level does not change by more than 40 meV upon hydrogenation, indicating that hydrogen is affecting primarily the interface dipole. The results, in general, conform to the predictions of Spicer’s unified defect model but the dependence of the detailed behavior on specific transition metals and the evidence for lateral barrier height variations show that some corrections are necessary.