In an extensive research program, reverse osmosis has been investigated for the treatment of wastewater containing heavy metals such as electroplating effluents where more stringent pollution control standards impose the use of very efficient treatment techniques, The present paper relates to the prediction of osmotic pressures for 2-1 and 2-2 single or mixed electrolytes. The experimental determination of the osmotic pressure using a spiral wound asymmetric membrane and the use of predictive literature models was executed simultaneously. The experimental results correspond very well with predictions using Fitter＇s model equations, while the other model equations overrate the osmotic pressure for concentrated electrolyte solutions.