The molecular origin of the non-linear viscoelasticity of polycarbonate (PC) has been studied by creep measurements at room temperature for three different states of physical ageing. The non-linearity of the isochronous stress-strain diagrams was analysed according to the Boyd-Jansson model. It was found that although the stresses (up to 27 MPa) caused marked non-linearity, they did not influence the physical ageing stage, as measured with a control load. The linear compliance component of the Boyd-Jansson model, in both longitudinal and transverse directions, was barely influenced by the physical ageing state of the PC, although the non-linear compliance component, in both longitudinal and transverse directions, changed considerably. It is proposed that the origins of the non-linearity of quenched polycarbonate at room temperature are stress activated changes in the alpha’ and beta mechanisms.