Ammonia chemisorption on a small and positively charged gallium arsenide cluster in the gas phase has been investigated using density-functional methods. The reactivity at various cluster sites are characterized in terms of binding energy and charge transfer from the adsorbate to the cluster with a strong correlation shown between the latter two : quantities. The calculations show a strong dependence of the reactivity of the cluster on its charge state. Ionizing the bare cluster to a cationic charge state enables multiple chemisorption of ammonia, which is much less favored by a neutral cluster and can be prohibited on a negatively charged cluster.