Hydrogenation is an efficient approach to tune the electronic, magnetic and chemical properties of single-layer hexagon boron nitride (h-BN). The relative stabilities and electronic properties of hydrogenated and protonated h-BN sheets are studied by means of density functional theory calculations. H and [Formula: see text] show very contrasting behaviors in chemisorption and clustering on h-BN, in which a single H atom prefers to adsorb on the top site of the boron (B) atom, and more H atoms tend to cluster on both sides of the h-BN layers in an alternating manner; while single [Formula: see text] prefers to stay on the nitrogen (N) atom, and protons are more likely to separate from each other on h-BN. The collective [Formula: see text] bonding feature of H-decorated h-BN lattice plays a key role in stabilizing the H clusters on the h-BN sheet. The non-magnetic H clusters with an even number of H atoms ([Formula: see text]) are energetically favored, compared with those with odd [Formula: see text]. Both the binding energy and band gap width vary in an oscillatory way as a function of [Formula: see text].