Gas composition is measured at the center of one-dimensional porous platinum/alumina pellets during carbon monoxide adsorption and catalytic oxidation. The apparatus is based on a single-pellet diffusion reactor that has been modified to allow continuous gas analysis and miniaturized in order to reduce the time constants of gas flow and mixing. Analysis of the CO adsorption response demonstrates that the centerplane volume and sample leak perturb the system only slightly and in a manner that can be accounted for during data analysis. A detailed kinetic model described previously is able to predict the qualitative features of the external concentration responses during carbon monoxide oxidation. However, the model is not able to predict major features of the responses measured at the pellet center, demonstrating that the reactor is able to provide stricter tests of kinetic models than reactors in which only external compositions can be measured.