The Saint‐Venant torsion problem of composite cylindrical bars with imperfect interfaces between the constituents is studied. Two kinds of imperfect interfaces are considered: one which models a thin interphase of low shear modulus and one which models a thin interphase of high shear modulus. In the former case, the traction on the interface is continuous but the axial warping displacement undergoes a discontinuity proportional to the axial shear traction. In the latter case, the warping displacement at the interface is continuous but the axial shear traction undergoes a discontinuity proportional to a differential operator of the warping function. The imperfect interfaces are characterized by certain interface parameters given in terms of the thickness and the shear modulus of the interphase. A derivation of these interface conditions is presented, and the Saint‐Venant torsion of cylindrical composite bars with both types of imperfect interfaces is formulated in terms of the warping function and in terms of a stress potential. An example of the application of imperfect interfaces is the construction of ‘neutral inhomogeneities’ in torsion problems. These are cylindrical inhomogeneities which can be introduced in a cylindrical bar without disturbing the warping function in it and without changing its torsional stiffness. Neutrality is achieved by a proper design of an imperfect interface with a variable interface parameter. Analytical expressions are derived for the variable interface parameter at neutral elliptical inhomogeneities in an elliptical bar. The paper concludes with a study of the decay of end effects in composite bars with imperfect interfaces. The simplest example of a concentric cylinder is chosen to illustrate that the decay length increases as the degree of the imperfectness at the interface increases.