Abstract Interdetector volume accounts for the time delay between detectors when more than one detector is used in size exclusion chromatography (SEC). In this work, interdetector volumes are determined from chromatograms for concentration detectors in series and in parallel, as well as for a parallel differential viscometer (DV)—differential refractometer (DRI) combination. The three methods examined for accomplishing this determination (peak apex, multipoint, and centroid methods) all provided equivalent results for truly monodisperse, low-molecular-weight compounds in all cases. For the DV-DRI combination, the interdetector volume obtained for the latter two methods increased with increasing molecular weight. This result was attributed to increasing peak skewness observed for DV chromatograms of narrow-molecular-weight standards as molecular weight increased. This occurred despite the fact that their normalized DRI chromatograms were all superimposable. In addition to helping to explain current widespread difficulties in determining interdetector volume, the observed effect also appears important in interpretation of narrow-molecular-weight distribution polymers. In Part II of this series we provide more evidence for the effect, examine its origins, and propose a solution.