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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[sub n]H[sup -] (n≤10) cluster anions produced by gas-feed Cs sputtering from different crystallographic forms of carbon display similar odd–even alternation with the even-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 n=10 in C[sub n][sup -]) in C[sub n]H[sup -] and compare it with our density functional calculations as well as observations in C[sub n]N[sup -]. We also observe that the size dependence of the abundance of C[sub n]H[sup -] clusters sputter ejected from a fullerene target exhibits a distinctly different power-law decline compared to crystalline and amorphous carbon. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]