The dissociative photoionization cross sections for the production of C+ ions are presented for the spectral region from the dissociative ionization threshold at 90 ¿. The cross sections are considerably larger than previously published data and should therefore influence the calculations of the C+ concentrations in the Mars and Venus ionospheres. It is suggested that the released energy in the C+ fragment ion could reduce the loss rate of the ions in collisional processes, thereby accounting for the apparent discrepancies between the observed and calculated C+ ion density in the Venus ionosphere.