We provide here an insight into the mechanism of the axial ligand exchange reaction between chlorosubphthalocyanines and phenols. Our combined experimental and theoretical results support a bimolecular s-bond metathesis mechanism in which the phenolic proton assists in weakening the boron-halogen bond concomitantly with substitution at the boron center. Such a reaction pathway, which is unusual in boron chemistry, is a consequence of the crowded and rigid chemical environment of the boron atom in these macrocycles. Furthermore, this work sheds light on the influence of different experimental parameters on the kinetics and efficiency of the most important reaction in subphthalocyanine chemistry.