This paper discusses the properties of the European Large Area ISO Survey (ELAIS) 15-μm quasars and tries to establish a robust method of quasar selection for future use within the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) framework. The importance of good-quality ground-based optical data is stressed, both for the selection of candidates and for the estimation of photometric redshifts. Colour-colour plots and template fitting are used for these purposes. The properties of the 15-μm quasar sample, including variability and black hole masses, are studied and compared to the properties of other quasars that lie within the same fields but have no mid-infrared counterparts. The two subsamples do not present substantial differences and are believed to come from the same parent population.