This work reports some observations made on particle motion in the transverse plane of rotating cylinders operated in the rolling regime. In particular, techniques were developed to estimate the transition between the active and stagnant layers. Based on the particle trajectory and velocity obtained, a new theoretical model based on the thin active layer approximation is proposed to estimate the size and shape of the active layer with respect to filling ratio, Froude number (rotational speed) and particle sizes. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.