The aim of this study is to highlight the impact of confinement effects in catalysis for reactants confined in small mesopores. Double bond isomerization of 1-hexene at 343 K has been chosen as model reaction and mesostructured aluminosilicates of the MCM-41 type with the very same chemical composition (Si/Al = 400 +/- 20) and varying pore sizes (2.3-9.3 nm) were used as catalysts. Adsorption isotherms of 1-hexene over analogous purely siliceous MCM-41 materials were measured at 343 K in order to determine the energetics of the adsorption process as well as levels of pores filling. Enthalpies of adsorption at zero coverage amounted to -55 to -47 U mol(-1). The isosteric heats of condensation were found significantly larger than the heat of condensation of 1-hexene in the bulk and increased as the pore size decreased. Over the fresh catalysts, the smallest pore materials proved the most active and the differences in activity correlate well with the sorption energetics. Over the deactivated catalysts, a sharp maximum in activity is observed for the material featuring pores 3.7 nm in diameter as the result of a compensation effect between diffusion limitations and confinement activity enhancement. (c) 2006 Elsevier B.V. All rights reserved.