A method has been developed to characterize a vibration source when coupled via resilient mounts to a receiver structure. This two-step measurement procedure can deliver the mobility and free velocity of a source, together with the mobility of the receiver to which it is connected, without decoupling the two structures. The method is feasible in a practical sense as it does not require any knowledge of mount properties. This is a major advantage as mount properties can deviate from their stated specifications through tolerances, and furthermore, the properties may change when loaded in the coupled-state. A benchmark test is used as a validation reference for the method where the properties of the resilient mounts are required and are assumed as known but not completely certain. The comparison of the benchmark and the principal method is used to illustrate the benefits of the latter given a small error in the supposedly known mount properties. In this first part of the paper, the principles of the two methods are illustrated using a simple demonstrator example while in the second part the feasibility of the method is further examined by virtual experiment involving two built-up plates resiliently connected at several points.