The air velocity has been measured in blast waves produced by the detonation of trinitrotoluene (t.n.t.) charges varying in mass from 30 to 200 000 lb. The technique consists essentially of using a high-speed camera to record the displacement of smoke trails formed close to the charge just before detonation. The initial decay of velocity behind the shock agrees well with theoretical predictions, such as those of Brode (1959), but at later times there is an extended outward flow, which, it is postulated, is caused by the 'after-burning' of the detonation products in the presence of atmospheric oxygen. It has been shown that this phenomenon does not occur in the case of the detonation of an explosive with a high oxygen balance, or for a nuclear detonation. The velocity decay within a t.n.t. blast wave may be described by the equation $V = V_s (1-\beta t) exp (-at) + a ln (1 + \beta t)$ fitted to the data by an iterative least squares procedure. It has been demonstrated that particle trajectories, determined by the smoke tracer technique, may be used to calculate the variations of density, pressure and temperature within the wave, without reference to other measurements.