The flow birefringence of the sulphuric acid solutions of poly- p-phenylene terephthalamide and poly- m-phenylene isophthalamide, and of 13 of their copolymers with differing molar contents of the components have been examined. The experimental optical shear coefficients Δn/ Δτ have been used to determine the number of monomer units S of the Kuhn segments in all the samples. The molecular models of the homo- and copolymers have been examined. A formula has been arrived at for the chain flexibility 1/ S as a function of the copolymer composition Z on the basis of the suggested models. The good correlation of the experimental with the theoretical 1/S- Z function confirms the additivities of the structural and deformation mechanisms of the polymer chain flexibilities.