A unique property of the free‐electron laser (FEL) is its capability to be tuned continuously over a wide spectral range. This is a major difference with all other high‐power lasers. However, the tunability of first‐generation FELs used to be quite poor (typically 10% or less), due to constraints imposed by the accelerator and the undulator. The free electron laser for infrared experiments (FELIX) uses an undulator with an adjustable gap, which permits wavelength scans over an octave in typically 2 min without the need for any readjustment of the electron beam. Results obtained in operation of the long‐wavelength FEL of the FELIX facility are presented. These involve measurements of the spectral range covered (16–110 &mgr;m), the output power, and the influence of the cavity desynchronism. The results are compared with numerical simulations.