We present a multi-wavelength catalog ( 15 \micron , R , K-band , 1.4 GHz flux ) plus spectroscopic identifications for 406 15 \micron 
sources detected in the European Large Area ISO Survey ( ELAIS ) region S1 , over the flux density range 0.5 < S _ { 15 \micron } < 150 mJy .
332 ( \sim 82 % ) sources are optically identified down to R \sim 23.0 .
Spectra or bona fide stellar identifications are obtained for 290 objects ( \sim 88 % of the optically identified sources ) .
The areal coverage , mid-infrared ( MIR ) and optical completeness of the sample are discussed in order to allow statistical and evolutionary analyses .
Two main spectroscopic classes have been found to dominate the MIR extragalactic population : z < 0.5 star-forming galaxies ( from absorbed to extreme starbursts : \nu L _ { \nu } ( 15 \micron ) \approx 10 ^ { 8 } -10 ^ { 11 } L _ { \odot } ) , which account for \sim 75 % of the sources , and Active Galactic Nuclei ( AGN ; both type 1 and 2 ) , which account for \sim 25 % of the sources .
About 20 % of the extragalactic sources are dust-enshrouded starburst galaxies [ e ( a ) spectra ] , and all the starburst galaxies appear more dust extincted in the optical than nearby normal galaxies .
We also identified 91 stellar objects ( \sim 22 % of the MIR sources ) .
The counts for starburst galaxies and AGN down to 0.6 mJy have been derived .
A general trend is found in the optical-MIR spectral energy distribution ( SED ) of the galaxies , where the MIR-luminous objects have larger MIR to optical luminosity ratios .
Based on a variety of analyses , we suggest that the ELAIS sources fainter than R \sim 23 are luminous and ultra-luminous MIR galaxies ( LIG-ULIGs ; \nu L _ { \nu } ( 15 \micron ) =10 ^ { 11 } -10 ^ { 12 } 
L _ { \odot } ) at intermediate redshifts ( z =0.5–1.5 ) , and that consequently the present sample is virtually 100 % spectroscopically complete up to z =0.5 .