The mass transfer coefficient in the high pressure channel in reverse osmosis, as widely used in design calculations at present, is for back diffusion due to concentration polarisation. Ramani introduced a new concept of ’negative mass transfer coefficient’ to represent the actual nett mass transfer from the bulk to the membrane wall. Correlations for the tubular configuration with turbulent flow which were developed by him have now been extended to the case of parallel planes which are more in commercial use. This is expected to entirely change the design methods of reverse osmosis plants.