Quantum Machine Learning (QML) has established itself as a robust statistical learning framework to infer quantum-chemical properties of molecules using relevant training data. Chemistry is a science rooted in chemical reactions, naturally involving multiple molecular species. Here, we extend QML’s capabilities to improve the prediction of quantum-chemical properties of chemical reactions by defining reaction representations - that are, representations taking as input multiple molecules, participating to a reaction, that are represented by their atomic identities and three-dimensional coordinates. Several reaction representations are constructed from established molecular ones are benchmarked on four datasets representative of thermodynamic and/or kinetic reaction properties. The hydroformylation barriers (hfb22) dataset (2,451 energy barriers) is also introduced as part of this work. The most relevant ingredients for designing a high performing reaction representation are extracted and used to construct the Bond-Based Reaction Representation (B2R2) for the prediction of quantum-chemical properties of chemical reactions. Finally, variations of B2R2 with improved scaling are provided.