The retention indices of 92 compounds, comprising four series of N 2-substituted N 1,N 1-dimethylamidines, R xN = C(R z)N(CH 3) 2, each containing the same set of 23 variable substituents at the imino nitrogen atom, have been measured on a non-polar GE SE-30 silicone gum rubber column. The values obtained for each series were correlated with those of corresponding model compounds, such as primary amines, R xNH 2, substituted hydrocarbons, R xH, ethyl esters of corresponding carboxylic acids, R zCOOC 2H 5, and dimethylamides, R zCON(CH 3) 2. Support is provided for an earlier conclusion that for the prediction of retention indices the correlation method should be used instead of additivity rules. It has been found that the regression parameter characterizing the influence of substitution at the imino nitrogen atom on the retention index depends on the type of substituent at the amidino carbon atom. On the basis of the retention indices of 115 compounds the parameters of an equation for the prediction of the retention indices ofN 1,N 1-dimethylamidines containing an alkyl or aralkyl substituent at the amidino carbon atom were found.