We have modeled the chemistry of hydrocarbon ions in the jovian ionosphere. We find that a layer of hydrocarbon ions is formed in the altitude range (300-400 \mathrm{~km}) above the ammonia cloud tops, due largely to direct ionization of hydrocarbons by photons in the wings of the (\mathrm{H}{2}) absorption lines in the 912- to 1100- (\AA) region that penetrate to below the methane homopause. We have explicitly included in the model 156 ion-neutral reactions involving hydrocarbon ions with up to two carbon atoms. Larger hydrocarbon ions are included as two pseudoions, (\mathrm{C}{3} \mathrm{H}{n}^{+})and (\mathrm{C}{4} \mathrm{H}{n}^{+}). The model shows that 15 reactions of (\mathrm{H}^{+}, \mathrm{CH}{3}^{+}, \mathrm{CH}{5}^{+}, \mathrm{C}{2} \mathrm{H}{3}^{+}, \mathrm{C}{2} \mathrm{H}{4}^{+}, \mathrm{C}{2} \mathrm{H}{5}^{+}), and (\mathrm{C}{2} \mathrm{H}{6}^{+})with hydrocarbon neutrals are the major processes that are responsible for the production and growth of (\mathrm{C}{1^{-}}, \mathrm{C}{2^{-}})and (\mathrm{C}{3^{-}})or (\mathrm{C}{4})-ions in the hydrocarbon ion layer. The model also shows that ions initially produced in the hydrocarbon ion layer are converted into hydrocarbon ions with more than two carbon atoms with very little loss by recombination. It is likely that successive hydrocarbon ion-neutral reactions continue to produce even larger hydrocarbon ions, so the terminal ions probably have more than three or four carbon atoms. In the auroral regions, the chemistry of hydrocarbon ions may modify the densities of neutral hydrocarbons, especially (\mathrm{C}{2} \mathrm{H}_{2}) in the upper mesosphere, and may play a major role in the production of polar haze particles. 1994 Academic Press, Inc.