High redshift BL Lac objects: spectroscopy of candidates

We report on 16 BL Lacertae objects that were proposed to be at z > 1. We present spectroscopic observations secured at the 10.4m GTC that allowed us to assess the redshift of these sources. In particular, for five objects we disprove the previous value of the redshift reported in literature and found that they lie at z 2.50. Based on the new high quality spectra we found that only half of the observed objects can be classified as bona-fine BL Lacs.