Catalysis plays a crucial role in chemical research and industry, yet its dynamics at the nanoscale has been little explored. Here, we use carbon nanotube transistors to measure in real time the catalytic activity of metallic nanoparticles. First, we present a method based on covalent nanotube functionalization to selectively attach a small number of nanoparticles on an individual carbon nanotube device. We demonstrate the covalent attachment of bimetallic Ru-Pt clusters and their aggregation into less than 10 nanoparticles (RuPtNP) on each nanotube device, using a combination of techniques including electrical spectroscopy and atomic force microscopy. Second, we monitor the catalytic transformation of dimethylphenylsilane in dimethylphenylsilanol with water in real-time, through changes in the nanotube electrical conductance. Upon exposure to silane, RuPtNP-decorated carbon nanotube devices show a rapid change in electrical conductance that decays slowly back to its initial state. We present the effect of silane concentration and electrostatic doping of the nanotube, and discuss possible mechanisms for the interaction between the catalytic reaction and the electrical signature. Carbon nanotube electronic sensors form a powerful tool to investigate various catalysis reactions, providing real-time monitoring over a broad range of time scales.