Since Sebera published his paper on Isoperms in 1994,much has changed. A significant amount of data has accumulatedon the mechanism of paper degradation, more activationenergies have been determined and there has been aconvergence of values. Assumptions about the relationshipbetween the relative humidity and the moisture content orthe rate of degradation of paper need no longer be made asnear complete theoretical descriptions are available. Themethod of modelling has therefore been revised and nowincorporates the Arrhenius equation and the moisture sorptionisotherm, which is best modelled by the Guggenheim-Anderson-deBoer (GAB) equation. This new method willallow direct experimental verification of the isoperm forcellulosic materials in museum collections. As the cost ofenergy usage for preser vation has now become an importantquestion, it is particularly critical to have access tomore accurate isoperms.