Topological insulators exhibit a metallic surface state in which the directions of the carriers’ momentum and spin are locked together. This characteristic property, which lies at the heart of proposed applications of topological insulators, protects carriers in the surface state from back-scattering unless the scattering centres are time-reversal symmetry breaking (i.e. magnetic) [1, 2, 3]. Experimental efforts to investigate this sensitivity of the surface state to magnetic scattering have so far focused on bulk doping topological insulators with magnetic defects [1, 2, 3]; in general this profoundly modifies the materials’ electronic structures, impeding the interpretation of results and obfuscating the specific role of spin-momentum locking. We identify a new method of probing the effect of magnetic scattering by decorating the surface of topological insulators with molecules whose magnetic degrees of freedom can be engineered independently of their electrostatic structure [4]. We show that this approach allows us to separate the effects of magnetic and non-magnetic scattering in the perturbative limit [5]. We thereby confirm that the low-temperature conductivity of SmB6 is dominated by a surface state and that the momentum of quasiparticles in this state is particularly sensitive to magnetic scatterers, as expected in a topological insulator.