An experimental and numerical study is performed of irreversibilities among particles in a wide-gap Couette device. Five spheres are placed in the Couette in a tightly packed arrangement. The inner cylinder is rotated five revolutions counterclockwise and subsequently five revolutions clockwise. Irreversibilities in the system are characterized by the radial spread of the particles measured by a radial moment. The experimental results are compared to a set of numerical simulations performed using a lubrication-corrected completed double layer boundary element method with a particle roughness model. In both the experiments and simulations, there is a critical value of the initial radial moment above which there is little irreversibility in the systems and below which irreversibilities accumulate rapidly. Particle roughness in the simulations can be used as a parameter to achieve the best agreement with experiment. It is noteworthy that the roughness value producing the best agreement is different than the actual average roughness of the particles used in the experiments. (c) 2006 The Society of Rheology.