The identification of a pressurized tank process is considered. A subspace identification method is employed that identifies the system in question from the Laguerre spectra of the input/output data. The method is presented in a general form and valid in the continuous and in the discrete-time case. Using the input/output Laguerre spectra for identification, instead of the complete input/output data vectors, leads to a considerable data reduction. Furthermore, it is shown by a simulation study that the subspace identification method n4sid works better in Laguerre domain than in the time domain.