Funding Information: We acknowledge financial support from the Spanish Agencia Estatal de Investigación (AEI) and the European Regional Development Fund (FEDER) (Grants Nos. PID2019-107338RB-C63 and PID2019-107338RB-C64) and the Academy of Finland (project numbers 311012 and 314882). We acknowledge the Paul Scherrer Institut, Villingen, Switzerland, for provision of synchrotron radiation beamtime at PEARL Beamline of the SLS and would like to thank N. P. M. Bachellier and M. Muntwiller for assistance. We thank R. Fasel and R. Widmer for provision of substrate and a sample holder for the synchrotron experiments. The authors thank the generous allocation of computer time at the computing center of DIPC. Publisher Copyright: © 2021 American Chemical Society. The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of five-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this alternative reaction path is responsible for the straight GNR growth in spite of the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants but also serve as a guide for the design of other precursor molecules.