This paper presents new population balance analysis to describe simultaneous coalescence and break-up in the formation of methylmethacrylate droplets in a batch oscillatory baffled reactor. It is concluded that the droplet data can very well be described by a model in which coalescence is taken to be shear induced, selection for break-up proceeds at a rate proportional to droplet volume and approximately four equally sized break-up fragments are produced per break-up event. It is shown that the experimental droplet size distribution data are self-similar in form and exhibit asymptotic behaviour characteristics also seen in the model. The coalescence rate is found to vary as the square of the oscillation frequency and the selection rate to vary with the oscillation frequency to the power five. As a result the asymptotic mean droplet volume is inversely proportional to the oscillation frequency. (C) 2003 Elsevier Ltd. All rights reserved.