Paper is treated as a member of the class of hydrogen-bond dominated solids for which the author derived a set of equations connecting E to the effective number of H-bonds per cm³, N, and to the parameters of this bond. In this paper it is shown that the effect of water on such solids is to reduce E in one of two modes or regimes. In regime (1), a unimolecular reaction leads to a simple breakdown of H bonds on addition of water. In regime (2), which begins when the regain exceeds a critical value equal to the B.E.T. monomolecular layer, the reaction is still unimolecular in N, but is complicated by a co-operative breakdown phenomenon as envisaged by Frank and Wen’s ‘flickering clusters’ concept. Equations derived using these concepts and a co-operative index, C.I ., based on Starkweather’s thermodynamic calculations, are checked against all data available to the author on paper, other cellulosics, Nylon 66 and wool with satisfactory results.