An expression for calculating the coupling loss factors between non-conservatively coupled substructures is derived based on the characteristics of the coupled substructures and the interface medium. The expressions consists of point mobility functions evaluated on the substructures near or at the junctions and a joint impedance function which represents the interface characteristics. The point mobility functions can be obtained analytically, for simple structures, or experimentally. The joint impedance is modeled by a dissipative stiffness element. The derived expressions for the coupling loss factors simplify to the well known expressions for conservative coupling when the dissipation term tends to zero. This approach is used to estimate the coupling loss factor between a shaft and a bearing liner with an oil film interface. The estimated results for the coupling loss factors are compared to experimental results obtained from a statistical energy analysis model. The estimated results and the experimental results show good agreement which verifies the approach used for the determination of the non-conservative coupling loss factors.