Phenomena related to the equilibrium take-up and retention of fluid in porous media can be explained and predicted in terms of the capillary sorption cycle (CSC) formed by the absorption and exsorption isotherms. An experimental method is described for determining the CSC as well as the simultaneous changes in the bulk volume of the porous medium as it cycles between 0 and 100% fill-up. The complex shape of the CSC and the CSC and the, sometimes highly significant, differences between the first and all subsequent cycles can be explained in terms of the mechanism which causes volume changes. The method of analysis provides a tool for investigating the capillary sorption properties of heteroporous media, such as fiber masses of different composition, structure, and bulk density. This has been demonstrated by applying the criteria of sorption equilibrium to phenomena related to the spontaneous fill-up and capillary transfer of fluid in the pore system of fiber masses