In a study of the characteristics of sulphonated polystyrene resin, essential to the separation of amino-acids, the cross-linkage, or water regain value, the diameters of the resin beads and the degree of sulphonation of the ion-exchange resin are shown to be important factors in the separation of individual amino-acids from their complex mixtures.A commercial automatic amino-acid analyser has been used to study the reaction between indanetrione hydrate and the eluted amino compounds passing through the reaction coil at various rates of flow. A special design of reaction coil has been used for the maximum development of the coloured complexes formed by the individual amino-acids, and the respective colour factors and molar extinction coefficients determined. The results serve to indicate the maximum rate of flow of the eluting buffer solution through the commercial reaction coil, above which the normal colour factors of the amino-acids are not obtained. The critical maximum flow-rate cannot be exceeded if the commercial instrument is to be used for the quantitative determination of amino-acids.