We have measured sets of mass spectra for positive ions produced by low-energy electron impact on phenanthrene. Ions have been mass resolved using a reflectron time-of-flight mass spectrometer, and the electron impact energy has been varied from 0 to 100 eV in steps of 0.5 eV. Ion yield curves of most of the fragment ions have been determined by fitting groups of adjacent peaks in the mass spectra with sequences of normalized Gaussians. The aim of this paper is to provide a detailed comparison of phenanthrene with its isomer anthracene, for which we have published results in a previous paper [1]. Appearance energies for a selection of fragment ions of phenanthrene have been determined, and are compared with anthracene. The most significant differences are observed in the ion yield curves of the ions containing 12 carbon atoms. The ion yield curves of phenanthrene have higher maximum yields and lower appearance energies compared to anthracene. For the fragments containing 9 and 10 carbon atoms the phenanthrene yields are slightly lower, but the appearance energies are the same as for anthracene. Small differences in yields are also observed for the fragments with 6 and 7 carbon atoms. The double and triple ionization energies of phenanthrene have been determined and are in agreement with anthracene.