The rotational particle separator is a patented technique for separating solid and/or liquid particles of 0.1 mu m and larger from gases. The core component is the rotating filter element which consists of a multitude of axially oriented channels which rotate as a whole around a common axis. Particles in the gas flowing in a laminar fashion through the channels are centrifuged towards the outer collecting walls of each individual channel while the purified gas leaves the channels at the exit. Solutions in closed form are presented for the probability that particles of given diameter are separated from the gas. Particle trajectories are governed by centrifugal forces and Stokes drag forces including Cunningham’s correction, Solutions are given for channels of the following cross-sectional shapes : concentric rings, circles, triangles and sinusoids. Account has been taken of parabolic (Hagen-Poiseuille type) velocity distributions inside the channels and Various distributions of the flow over the assembly of channels. The results compare favourably with measurements executed on six differently sized practical versions of the rotational particle separator.