The gas density in front of a magnetron cathode has been found to be significantly reduced as a function of magnetron current. The reduction is due to gas heating and rarefaction resulting from collisions with the sputtered atoms ejected from the cathode surface. The density reduction is nonlinear with discharge current, and is sensitive to the sputter yield of the cathode, the average ejection energy of the sputtered atoms, the cross section for sputtered atom–gas atom collisions and the pressure, temperature, and thermal conductivity of the background gas. The process can be treated semiquantitatively in terms of heat conduction from the hot region near the cathode to the cold walls of the chamber. The presence and magnitude of this gas density reduction should have significant impact on the understanding of the plasma impedance, as well as the deposited film properties and the relative arrival rates for reactive depositions.