A study has been conducted on the origin of the molecular weight (MWT) dependence of the apparent interdetector-delay (IDD) in multidetector size exclusion chromatography (SEC). The IDD-MWT relationship proved to be independent of polymer concentration (for a given molecular weight). In contrast, the IDD-MWT relationship was found to be influenced by the polymer-solvent interaction, as shown by SEC experiments on polystyrene in a mixed tetrahydrofuran-water eluant. Confirmation of this discovery was provided by measured variations in the IDD-MWT relationship for polystyrene and poly(methyl methacrylate) in THF eluant. possible reasons for the strong influence of the polymer-solvent interaction on the IDD are discussed in the paper.