We present a catalog of optical spectroscopic identifications of sources detected by Spitzer at 3.6 or 24 µm down to ∼10 and ∼280 µJy, respectively, in the SWIRE/XMM-Newton/ELAIS-S1 field and classified via line width analysis and diagnostic diagrams. A total of 1376 sources down to R ∼ 24.2 mag have been identified (1362 detected at 3.6 µm, 419 at 24 µm, and 405 at both) by low-resolution optical spectroscopy carried out with FORS2, VIMOS, and EFOSC2 at the Very Large Telescope and 3.6m ESO telescopes. The spectroscopic campaigns have been carried out over the central 0.6 deg 2 area of ELAIS-S1 which, in particular, has also been observed by XMM-Newton and Chandra. We find the first direct optical spectroscopic evidence that the fraction of active galactic nuclei (AGN; mostly AGN2) increases with increasing F (24 µm)/F (R) ratio, reaching values of 70(±20)% in the range 316