Abstract: We define homologous retention index I_h as a retention index obtained by using the homologous series of the substance to be characterized by GCas reference standards. Based on the analysis of hierarchial data with five sources of variation, we have come to the conclusion that the relative standard diviations of the proposed homologous retention index caused by the factors of concentration of stationary phase and column temperature are much smaller than those of relative retention and Kovats retention index. Therefore homologous retention index is most precise, and can be used more reliably for characterization purposes, and is expected to find broader applicability. Kovats retention index may be looked on as a special case of homologous retention index when applied to paraffin hydrocarbons.