The yields of excited singlet states formed in some liquid hydrocarbons are reported for exposure to synchrotron X-rays of energies between 5 and 14 keV. The yields are derived from measurements of ultraviolet fluorescence emission of the hydrocarbons. Results reported are relative to the yield of excited singlets in liquid benzene. The yields decrease with decreasing X-ray energy, and at the lowest energies studied G(S-1*) = 1.5/100 eV for cis- and trans-decalin. Results are compared to computer simulations.