RbCn cluster anions have been produced by Rb sputtering of graphite. The intensity ratio of clusters with an even number of carbon atoms to those with an odd number of carbons is much larger for RbCn- clusters than for C-n(-) clusters. Ab initio quantum mechanical calculations suggest that this arises from RbCn electron affinities that are close to zero or negative for odd n, rather than from an enhanced odd-even alternation in the affinities.