The diffusion of three series of homologous solvents (i.e. alcohols, ethers and esters) in polyurethane block copolymers was analyzed on the basis of a new fundamental and numerical approach of mass transfer in the transient regime. Accounting for three complementary effects-diffusion, convection and polymer swelling-the mass transfer numerical model enabled to determine diffusion coefficients which were much more representative of the solvent intrinsic properties than those obtained by the simple Fickian approach which ipso facto globalized different subsidiary effects including convection and swelling. Correlations were then determined between the parameters describing the diffusion coefficient (e.g. intrinsic diffusivity and plasticizing constant, etc.) and physical chemical characteristics of the diffusive species (e.g. dynamic size, polarity parameter, etc.). Following that approach, it became possible to account for the diffusion coefficients of linear or branched species of the same type (e.g. alcohols, ethers or esters) by the same structure-property relationship, thus overcoming one common limitation of former works on diffusion through polyurethanes for which two different types of diffusion laws had been reported systematically for linear and branched species, respectively. (C) 2004 Elsevier B.V. All rights reserved.