The mechanism of enhanced transparency (brightening) of a layer of carbon and magnesium oxide particles of submicron size moving at a velocity of 1200 m/s at a reduced ambient pressure of 50 Pa is investigated. The layer of particles was exposed to a HF cw laser radiation with power density in the beam up to 30 MV/m2 and beam cross section of 10 cm2. The transparency is shown to be enhanced first of all due to removal of particles from the field of high–power laser radiation. Transformation of the dispersed particle composition after exposure to laser radiation has been evaluated.