Measurements are described of the reflexion and transmission of 3 cm wavelength microwaves b y detonation waves in stoichiometric oxyhydrogen seeded with nitrogen, and oxyacetylene mixtures. Conductivity measurementsare also obtained by means of two types of d.c. probe, termed ‘axial’ and ‘gap ’ probe respectively. The observed reflexion coefficient in the oxyhydrogen waves is approximately tenfold greater than the calculated value assuming a step profile and equilibrium ionization at the detonation front. The character of the gap probe records indicates that regions of higher ionization occur along the detonation tube axis at locations which correspond to the intersection points o f the oblique shock waves generated b y the wavefront and the w all boundary layer. It is shown that the enhanced power reflexion can be explained in terms of these regions. Microwave absorption and d.c. axial probe measurements give values of conductivity in reasonable agreement with theory, assuming the absence of OH ions. The measured values o f reflected pow er and conductivity in oxyacetylene waves show no anomaly for initial gas pressures in the range 760 to roughly 300 m m H g. A t lower pressures the reflexion coefficient is w ell in excess o f the calculated value and both m icrow ave and d.c. probes exhibit a frontal ionization peak. This is in accord with the observation, by other investigators, o f chemiluminescence in the reaction zone of waves in organic fuel mixtures and chemi-ionization process in the reaction zone of flames. A value for the ionelectron recombination rate constant is derived which is in agreement with that observed in flames.