The importance of C-F...H-C interactions in aligning molecules of the ll-trifluoromethyl derivative of cyclopenta [a] phenanthren-17-one in the crystalline state has been investigated by a comparison of the crystal structures of the 11-trifluoromethyl derivative with those of the previously reported 11-methyl and unsubstituted analogues. In the crystal structure of the 11-fluoro compound there are three C-F...H-C interactions and one C=O...H-C interaction per molecule, while for the methyl derivative one C=O...H-C interaction is all that serves to align molecules, and the molecular packing is different from that of the 11-trifluoromethyl derivative. These results imply that such weak intermolecular interactions are important in aligning molecules when stronger interactions such as O-H...O hydrogen bonds are not possible, and that such interactions are similar in importance to the ring-ring interactions that result in a 7.5 Angstrom unit-cell dimension. The arrangements of interactions in each crystal structure have been highlighted by the use of graph-set analysis. The graph-set description of the packing of the 11-trifluoromethyl derivative in crystals may be described as N-1 = R(2)(2)(14), R(2)(2)(14), C(9), R(2)(2)(16), C(7), C(8), C(8). With respect to the C-H...O interactions in the 11-methyl and the unsubstituted hydrocarbon the graph-set analyses are C(7) and C(11) respectively, compared with the C(8) in the ll-trifluoromethyl derivative. Thus, for each compound, the hydrogen atom involved in the C-H...O interaction is different. The molecular structure of the 11-trifluoromethyl derivative of cyclopenta[a]-phenanthren-17-one shows bay-region distortions similar to that found in the analogous 11-methyl derivative. The molecular geometry of the 11-trifluoromethyl compound was compared with that for the same ring system with an electron-donating CH3 substituent replacing the electron-withdrawing CF3 group. The crystal structure of the unsubstituted compound is also used in the comparison. It is found that for the trifluoro derivative there is a lengthening of the bonds in the bay region and also of the bond adjacent to the site of substitution. Other bonds acquire, as a result, more double-bond character.