This Technical Report describes a procedure, based on a diffusion model, to be applied to the estimation of specific migration of substances into drinking water from organic materials intended to come into contact with drinking water. The modelling approach is readily applicable to certain organic materials, as explained in this report. In principle, the diffusion modelling approach is applicable to other organic materials but practical difficulties, in relation to obtaining data to feed into the diffusion model, may restrict or prevent its application. Accordingly, in addition to the diffusion model, scientific estimation procedures for the required data inputs need to be considered. The approach is normally applicable to organic substances that are soluble in the material matrix. Substances applied externally to a product made of an organic material, e.g. antistatic agents, lubricants, etc. are excluded from the diffusion modelling approach, as are electrolytes, salts, oxides and metals. Only organic substances with well-defined molecular weight or mixtures with well-defined ranges of molecular weights are amenable to the diffusion modelling approach. The diffusion modelling approach is readily applicable to amenable organic materials in the form of a pipe or a sheet, where data such as material thickness is readily calculable. More complicated product shapes, such as fittings, require assumptions to be made. It may not be possible to model the effects of test waters that are chemically active, for example test waters to which chlorine has been added to simulate chlorinated drinking water. This is because substances that migrate from a material into water containing chlorine can be converted by chemical reaction into substances with different properties.