Effect of carbon nanotubes (CNT) crystal structure on chemical activity (SAR) has been studied extensively in CNT suspension. However, these effects are experimentally underexplored for gas-CNT interactions (heterogeneous catalysis). We report about ultra-high vacuum kinetics experiments on metallic, semiconducting, and mixed CNTs. A weak and probe molecule specific SAR is present for adsorption inside the CNTs but not for the population of external sites. The experimental data are in part consistent with theoretical predictions. (C) 2009 Elsevier B. V. All rights reserved.