The effect of different calcium levels on the permeation rate and on the structure of deposits formed in reverse osmosis (RO) was studied as a function of the transmembrane pressure difference. An almost complete removal of the soluble calcium together with the other low molecular constituents by diafiltration significantly changed the deposited protein layer compared with untreated skim milk. By determination of the flow resistance of the deposited layer Delta(pL) it was concluded that by the accumulation of individual micellar casein particles a loose structure was formed. The removal of the total calcium, i.e. both soluble and the calcium bound in the casein micelles, by ion exchange resulted in the disintegration of the micelle structure, causing a very dense deposited protein layer consisting of casein submicelles and a significant reduction of the permeation rate compared with untreated skim milk.