In the field of quantum spin liquids 1-3, the honeycomb magnet α-RuCl3 has attracted considerable interest because it is proximate to the Kitaev Hamiltonian HK 4-12. In 2006, Kitaev 13 published the exact solution for the ground state of HK, and found that the excitations are Majoranas and vortices. The thermal Hall conductivity κxy of Majorana fermions is predicted to be half-quantized 13. This effect has been reported in α-RuCl3 17,18, but this observation has proven difficult to reproduce 19,20 and remains highly controversial. We report detailed measurements of the Onsager-like planar thermal Hall conductivity κxy in α-RuCl3, a spin-liquid candidate of topical interest. With the thermal current JQ and magnetic field B||a (zigzag axis), the observed κxy/T varies strongly with temperature T (1-10 K). Using a novel fitting procedure, we show that this temperature dependence matches the distinct form expected for topological bosonic modes in a Chern insulator-like model. Our analysis yields band energies that are in agreement with earlier experiments and that the excitations evolve into increasingly magnon-like modes at high B with a Chern number ≈ 1. The bosonic character is incompatible with half-quantization of κxy/T.