Abstract: It is one of the important development in theory of physicochemical analysis ofdrugs to select the optimal combination of TLC system by using mathematical means.Besides the numerical taxonomic technique with information content as selection crite-rion,the taxonomic technique of information content of system-difference shows unique advantage in its theory and application. An investigation of the latter technique applied in the optimal selection of 56 kinds of TLC systems of 22 kinds of sulfa-drugs and a preliminary interpretation of the difference between these two techniques have been made.