  We present optical identification and characterization of counterparts of
four objects previously catalogued as ultra-luminous X-ray sources. The objects
were selected from the Colbert & Ptak (2002) catalogue. The optical
counterparts are identified as point-like objects with magnitudes in the range
\~17-19. The optical spectra of three of the sources (IXO 32, 37 and 40) show
the presence of emission lines typical of quasars. The position of these lines
allows a precise estimation of their redshifts (2.769, 0.567 and 0.789 for IXO
32, 37 and 40 respectively). The fourth X-ray source, IXO35, is associated with
a red object that has a spectrum typical of an M star in our Galaxy. These
identifications are useful for building clean samples of ULX sources, selecting
suitable targets for future observations and performing statistical studies on
the different populations of X-ray sources.
