Different membrane manufacturer software are available for the designers of RO plants. Software releases are reissued periodically and distributed to the designers to make simulation of membrane behaviour. If you are familiar with these software you know that, a part the different products characteristics, depending on the membrane specification, the acid dosage requests to modify the pH of the feed water are very different for each supplier and often also for each software release. In the paper, using a data input a typical sea water Mediterranean Sea analysis, the membrane software are used to simulate different design conditions and the acid dosage figures are compared with those obtained from an original model developed by the authors, based on the carbonate salts equilibrium.