The Jülich double‐monochromator system has been brought into operation at the Jülich Isochronous Cyclotron. The results of the ray tracing calculations as well as the tests with the cyclotron beam show that it has a momentum resolution power R = p o /Δ P FWHM = 20000 and a transmission of about 2 % for 1 mm slit‐width settings at the entrance and the exit of the system. These values are obtained for a horizontal and a vertical emittance of 20 and 25 mm·mrad, respectively, and a momentum bin width of 1.5/1000 of the cyclotron beam.