A high temperature rotary Knudsen cell assembly is described which, operating in conjunction with a mass spectrometer, may be used to determine the spatial distributions of individual species effusing from ideal and nonideal orifices and of species vaporizing from surfaces. This unit rotates about an axis contained within the plane of the face of the orifice or surface and is capable of reproducible angular settings of 0.2° under high temperature, high vacuum conditions. The apparatus is particularly suited to those types of mass spectrometers which are too cumbersome to be used as movable detectors. The angular distribution curves which result from these measurements may be used to determine orifice transmission probabilities and, in certain cases, to determine mass spectrometric parent‐fragment relationships which other techniques have failed to resolve.