To understand the effect of rock types on the fractionation of light hydrocarbons in natural gases during migration in sedimentary rocks, the retention time of each hydrocarbon (C₁, C₂, C₃, i-C₄, i-C₅ and n-C₅) was measured using gaschromatografic columns packed with various rock and mineral samples. The fractionation occurred when the samples contained expandable clay minerals or zeolites. The order of retention times depended on the kind of clay minerals or zeolites present in the column. When montmorillonite, halloysite, vermiculite and mordenite were used, the order was C₁ < C₂ < C₃ < i-C₄ < n-C₄ < i-C₅ < n-C₅. When clinoptilolite was used, the order was C₁ < i-C₄ < C₂ < i-C₅ < C₃ < n-C₄, probably because the pore size of clinoptilolite is somewhere between the cross-sectional diameters of n-alkanes and i-alkanes. The fractionation became larger as the columns packed with mineral samples aged. When not aged, only small fractionation was observed. These results show that the dehydrated state of the interlayer spaces of clay minerals or the pore spaces of zeolites play an important role in fractionation. Furthermore, these results suggest that the large fractionation by clay minerals or zeolites does not occur under normal subground condition where these minerals are hydrated.