A model of two-layer flow over a broad crested weir is used as the basis for comparing the performance of weir type skimmers. It is shown that the oil concentration of the mixture flowing over the weir can be described in terms of a single densimetric Froude number based on the weir flow and width, the slick thickness, and the density difference between the oil and the underlying water. Although all the data sets exhibit a large scatter, they broadly follow the trend of the model, and the analysis clearly demonstrates the superiority of the hydro-adjustable designs. (C) 2002 Published by Elsevier Science Ltd.