The existence of complex and highly variable natural organic materials (NOMs) in virtually all potable supply source waters is broadly recognized. It is also general knowledge that granular activated carbon (GAC) adsorbers, integral parts of many potable supply treatment systems, effectively adsorb various molecular fractions of such NOMs. There is strong evidence that the phenomenon of ‘preloading’ of GAC by NOM can have profound effects on its subsequent ability to remove organic contaminants. Such effects on the performance of adsorbers in their primary role of contaminant removal are, nonetheless, seldom factored adequately into their initial design. The objective of this paper is to present a rational approach, a blend of mechanistic modelling and empirical parameter estimation, by which the impacts of NOM preloading can be incorporated into the design of GAC systems on a source-water specific basis. The results of a series of investigations from which a quantitative approach to the integration of preloading effects in the design of such adsorbers has evolved are detailed, and a specific case study application of the approach is described.