We have studied the interaction of hydrogen with small neutral and anionic carbon clusters using density functional calculations. The geometry, stability, and electronic structure of these clusters show an odd-even alternation originating in the bonding nature of the carbon atoms. Our mass spectrometric measurements of the abundance of C n H Ϫ (nр10) cluster anions produced by gas-feed Cs sputtering from different crystallographic forms of carbon display similar odd-even alternation with the

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... n-n clusters being relatively more abundant. The calculated trend in the adiabatic electron affinities shows a behavior similar to the experimental abundance pattern. We discuss a possible partial suppression of the chain-to-ring transformation ͑which normally occurs at nϭ10 in C n Ϫ ) in C n H Ϫ and compare it with our density functional calculations as well as observations in C n N Ϫ . We also observe that the size dependence of the abundance of C n H Ϫ clusters sputter ejected from a fullerene target exhibits a distinctly different power-law decline compared to crystalline and amorphous carbon.