Study of velocity fluctuations observed by means of ionization probes during the development of detonation reveals its significance as an indicator of physical characteristics of the flame. The scatter in time of arrival (the reciprocal of the velocity) was found normally distributed at an 85% probability level. The means and standard deviation were determined within 5% and 20%, respectively, at a confidence level of 90%. The intensity of scatter is interpreted consequently as indicative of the combustion front fluctuation that can be considered to delineate the ``effective flame thickness.'' It is found then that, as the flame accelerates, its effective thickness first increases, reaching a maximum in the vicinity of velocity overshoot, and then decreases, attaining finally a minimum, constant value when the steady detonation wave is established. An interesting bimodal distribution of scatter for the 2H2–02 mixture has been observed, indicating a possible existence of two alternative, independent modes for the development of the process.