Substructure-decoupling techniques are used to identify a substructure as a stand-alone system while it is coupled to a complex structure. These techniques can be used for various applications, e.g., when the substructure cannot be measured separately from the complex structure, when modal testing methods are not appropriate due to the limits of the measurement equipment and for vibration-control techniques. The complex structure consists of the unknown substructure and the remaining structure. A drawback of the available substructure-decoupling techniques is that they require a model of the remaining substructure. However, when the model cannot be calculated or (experimentally) identified, the substructure-decoupling techniques cannot be used. In this paper a new approach is presented that does not require a model of the remaining substructure, but is based on an experimental identification of the interface forces. The sensitivity of the approach to experimental errors was researched. Numerical and experimental test cases are researched.