We present here the size-dependent decomposition of methanol (MeOH) over narrowly distributed Pt nanoparticles supported on nanocrystalline anatase TiO 2 powder. Micelle encapsulation has been used to create Pt catalysts with average particle sizes of ~ 4, 6, and 8 nm. A packed bed mass flow reactor and mass spectrometry were employed to quantify the catalyst's activity and selectivity. Among the catalysts tested the smallest nanoparticles showed the best performance including an onset reaction temperature of ~145 °C. No byproducts such as CO 2 or CH 4 were observed in the test range of 100 to 330 °C.