The electronic and geometric structure of various substituted borylenes BR (where R = H, F, Cl, Br, CH(3), Ph, NH(2), NHMe, and NMe(2)) in their lowest singlet and triplet electronic states was investigated by computational means using hybrid density functional (B3LYP) and second-order Møller-Plesset perturbation theories combined with 6-311+G** and cc-pVTZ basis sets. The reactivity of singlet borylenes towards prototypical saturated and unsaturated hydrocarbons was examined by the MP2 method in conjugation with the cc-pVTZ basis set and also by coupled cluster [CCSD(T)] computations in combination with the aug-cc-pVTZ basis set. To study the energetics and the mechanism of the addition reaction of borylenes to unsaturated CC bonds, ethyne and ethene are chosen as model compounds. The insertion reaction of borylene into a C-H bond of methane was also investigated. The addition reactions of borylenes to multiple C-C bonds are strongly exothermic. In case of the BH molecule the reactions proceed without barrier and are the most exothermic. For the insertion reaction of borylenes into methane, two approaches could be identified. Again, the smallest reaction barriers and highest reaction energies were computed for the BH insertion, while the highest barriers and the smallest exothermicities were obtained for the BF molecule. On the basis of frontier molecular orbital energies, barrier heights, reaction energies, and transition state geometries BH is the most electrophilic borylene, followed by BPh, while aminoborylenes and BF are the most nucleophilic ones among the investigated derivatives. Accordingly, reactions of BH have the smallest barriers (if there is one at all) and the largest reaction energies, while the reactions of BF have the highest barriers and the smallest reaction energies.