The influence of coplanarity in dyes on the dyeing properties of nylon-6 fibre has been studied with o-Tolidine -Acid and m-Tolidine R-Acid. When nylon-6 fibre was dyed with color acids of the o-Tolidine -Acid and the m-Tolidine -Acid, it was nearly same in standard affinity, standard heat of dyeing and standard entropy change. It was also found that the entropy change on dyeing in the case of the m-Tolidine R-Acid was greater than that of the o-Tolidine -Acid. The non-coplanarity of the m-Tolidine R-Acid did not lower the standard affinity of the dye to nylon-6 fibre. This behavior contrasted with that of sterically hindered direct dyes to cellulose fibres(Benzopurpurine 4B and m-Benzopurpuring 4B to cellulose fibres). But in the case of sodium salts of the o-Tolidine -Acid and the m-Tolidine R-Acid, the standard affinity of sterically hindered m-Tolidine R-Acid was lower than that of the o-Tolidine -Acid.