The retention indices of series of dimethylformamidines, dimethylacetamidines, tetramethylguanidines and corresponding primary amines and also substituted hydrocarbons on a GE SE-30 column were determined. Limitations of the additivity rule in the prediction of retention indices are discussed and the correlation method is proposed instead. It was found that the prediction of retention indices of compounds containing an amidino group, based on the retention indices of the corresponding substituted hydrocarbons, may lead to erroneous results. Much better results are obtained if the retention indices of the corresponding primary amines are used for this purpose.