A method for decomposition of an experimental signal that contains several normal modes with a prescribed frequency, is proposed. The method is based on an expansion of solutions for small‐amplitude spatially growing disturbances in a biorthogonal eigenfunction system. It is shown that data of one velocity component at a given cross‐stream section of the flow are in some cases, sufficient for the decomposition. The method is applied successfully to analyze small‐amplitude periodic disturbances in a transitional wall‐jet flow.