This article about revers osmosis has been written to given a possible background for a way to introduce it as a unit operation. Naturally the principle today costs too much money to be introduced as a deperate unit in process engineering, but it has together with other values, immidiate use. The article was given as a lecture at Scandinavian Congress of Chemical Engineering in Copenhagen on the 4th of November 1971. The presentation of the work at the congress is given in an appendix.