Dissolved air flotation (DAF) is often used after a primary gravity separator to enhance the quality of wastewater, so it can be released to streams, rivers or the sea. The main aim of the DAF experiments reported here was to measure the oil droplet removal efficiency (eta) mostly in the range 15-80 mu m from oil-in-water mixtures. The DAF tank used in this investigation was a scale model of real DAF unit. Two kinds of oil, vegetable and mineral and two types of water, fresh and salty were used, and four other operating parameters were varied. A droplet counting and oil-in-water measuring methods were used to estimate the eta. Dimensional analysis concluded that the eta in this experiment is a function of eight other dimensionless groups and the experimental data has been subjected to multivariable linear regression. The resulting correlation was found to have a root mean square error of 6.0%, but predict eta outside the range zero and one. An alternative mathematical formulation was devised that cannot predict eta outside the range. Regression of the data by this formulation, which had the same number of adjustable parameters as the linear regression, was successful with a lower root mean square error of 5.5%. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.