Mooring systems employed for floating structures commonly result in non-linear load-excursion characteristics. The non-linear stiffness and the subsequent amplitude-dependent natural frequency influence the vortex-induced motion of the structure. The application of linear stiffness vortex-induced motion modelling and experimental data has been shown to produce significant uncertainties regarding the onset and vortex-induced response prediction of catenary moored cylindrical structures (Bjarke et al. 2003; Dijk et al. 2003). In the present study, the two-degree of freedom vortex-induced motion of non-linearly compliant elastically mounted rigid cylinders was experimentally investigated. Specifically, third-order polynomial hard-spring stiffness, typical of catenary moorings was considered. The linear and cubic compliance components were independently varied over the non-linear compliance ratio range of 0 to 0.29. The study revealed that the analysis of non-linear compliant structures assuming the corresponding linear system response is conservative with respect to mean inline structure displacement and the maximum inline and transverse oscillatory response. Relative to the non-linear compliant system, the transverse and inline motions were over-predicted by adopting a linear restoring force model. The vortex-induced vibration initial lock-in conditions were also unaltered with increasing non-linear stiffness. Conversely, the analysis of non-linear compliant structures, assuming the corresponding linear system response, is not conservative with respect to the lock-out point and the maximum structure excursion (and consequently the maximum mooring line tension experienced) over the lock-in range. This has potential bearing on the way in which highly non-linear compliant systems are modelled with regard to their vortex-induced motion response.