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The degree of homogeneity of soil parent materials is estimated by similarity index method in 4 pedons from Jafarabad, Arnirabad, Karaj, and Shurghaleh series in Karaj-Abyek area. In order to determine the similarity Indices the soil samples have been treated for carbonate, gypsum, organic matter and soluble salt removal in the laboratory. After these eliminations, sand and silt fractions(20-2000 micron) have been separated into nine different size fractions by wet seiving(i.e.,1000,500,345, 250,177,125,88,63,2omicrons). The relaitive weight and weight percentages of different

fractions were calculated based on oven dried samples at 105 Computation of the
indexes is based on summation of the minimum weight percentages of nine fraction groups between two adjacent samples. The index of 100 or close to 100 means a high similarity, whereas the index of 0 or low means no or very little similarity. The indexes obtained for the studied profiles ranges between 41.9 to 95.4(both for Karaj serie). Indexes are generally very low but mostly in a good agreement with the litho logical discontinuities. There are however, some discrepancies such as in the surface horizon of Jafarabad profile and the whole profile of Shurghaleh serie. Considering the Jafarabad profile, the dissimilarity is probably due to mixing of.the surface material as a result of sedimentatioEl and/ or erosion. Therefore, the sirnilarity index is probably more accurate. In Shurghaleh profile, the difficulties in determination of the amount of primary and secondary gypsum and carbonates are probably the main reasons for non-accurate determination of lithological discontini uities.