A combined experimental and theoretical study of the reactivity of small boron cluster cations with HF is reported on in this article. Cross sections for the reaction of mass-selected boron (B-n(+), n It =2-14) and boron monohydride (BnH+, n=4, 7, and 10) cluster ions with HF are reported for collision energies from 0.1 to 10 eV. The reaction proceeds via a chemically bound intermediate complex, and the major product channels involve elimination of BF or H from the BnHF+ intermediate. Both reactivity and product branching depend strongly on cluster size. Reactivity at low energies generally decreases with increasing size, with the appearance of significant bottlenecks to reaction. The effect of adding a hydrogen atom to the reactant clusters was examined for several cluster sizes. In contrast to the boron oxides, hydrogenation of elemental boron cluster ions has little effect on reactivity with HF, suggesting that H does not compete effectively for HF binding sites. The results provide a sensitive test for the accuracy of ab initio calculations on B-n(+) and BnH+ and BnF+ and the calculations provide insight into the reaction mechanism and effects of cluster structure on reactivity.