The authors present the first results on the optical identification of 89 ROSAT sources on the second Lockman hole. Because of the low Galactic H I column density towards this region the X-ray spectra of the sources are relatively unaffected by Galactic absorption, and a relation between X-ray spectral properties and the nature of the sources can be established. The authors have been able to classify 51 of the sources (57%) as stars, optically extended galaxies or QSOs, on the basis of R-band CCD images, optical spectroscopy of the optically brightest sources, and colour information from APM measurements of the POSS plates. They find that the fraction of the X-ray sources in the sample identified with galaxies is ⪆10%. Similarly ⪆29% are QSOs and ≅10% are stars. The galaxies form a heterogeneous group of sources, including ellipticals without emission lines, normal star-forming, starburst and broad emission line galaxies. The X-ray spectra of the various types of galaxies are on average harder than those of the QSOs and more similar to the X-ray background (XRB). This suggests that galaxies may make a significant contribution to the soft X-ray source counts at faint flux levels and in the synthesis of the soft XRB.