The influence of hydrogen concentration on the characteristics of fully developed self-sustained detonation waves propagating in H2—CO—O2mixtures at one atmosphere initial pressure was experimentally investigated by means of Q-switched pulsed laser schlieren photography. The main aspects of interest were the reaction zone thickness and the detailed wave front structure. The concentration of CO in the fuel ranged from zero to 97.5 % by volume; pure CO—O2mixtures did not detonate. A rotating prism Q-switched laser was modified to achieve the synchronization purpose, thus the need for an expensive electro-optical Q-switching device was eliminated. The laser schlieren photographs indicate that the wave front becomes nonplanar and the spacing between the transverse waves becomes larger as H2is replaced by CO in H2—O2mixtures. It is suggested that weak shocks, produced by secondary explosions, are the propagating mechanism for the trailing portion of the transverse waves. Experimental results show that the reaction zone thickness increases sharply with increasing CO concentration only when the CO concentration exceeds about 75% of the total fuel. A simple relationship between the reaction zone thickness and the hydrogen concentration in the mixture is formed. It is also demonstrated that the reaction zone thickness is affected by the channel size; it becomes thicker in smaller channels.