The application of ionization gap or ``pin'' probes to measure flame velocities during the development of detonation is presented. Probe construction and circuit techniques are described which, despite the widely varying character of the ionization signals in this region, yield timing precision of 2% or less for hydrogen‐oxygen mixtures. Analysis of the ionization probe circuit permits the identification of the probe signal with the ionization levels encountered in the various waves generated during the development of detonation. In this manner it is possible to differentiate between shocks, flames, and detonation waves and to determine the axial flame velocity profile.