Shock structure during ionization of a hydrogen‐helium mixture has been followed using hydrogen line and continuum emission measurements. A reaction scheme is proposed which includes hydrogen dissociation and a two‐step excitation‐ionization mechanism for hydrogen ionization by atom‐atom and atom‐electron collisions. Agreement has been achieved between numerical calculations and measurements of emission intensity as a function of time for shock velocities from 13 to 20 km/sec in a 0.208 H2‐0.792 He mixture. The electron temperature was found to be significantly different from the heavy particle temperature during much of the ionization process. Similar time histories forH&bgr;and continuum emission indicate upper level populations of hydrogen in equilibrium with the electron concentration during the relaxation process. The expression for the rate constant for excitation of hydrogen by atom‐atom collisions that best fit the data waskAA = 4.0×10−17 (8kT/&pgr;&mgr;)1/2 exp (−10/kT) cm3 sec−1, where it has been assumed that the excitation cross section is the same for hydrogen and helium collision partners. The electron‐atom excitation rate constant,k8 = 7.5 × 10−16 (8kTe/&pgr;&mgr;e)1/2 exp ( −10/kTe), determined from this investigation, was in agreement with recent electron beam cross‐section measurements.