Significance An explicit formula is presented for the effectiveness factor of a catalyst particle in the form of a hollow cylinder taking into account the reactant diffusion in the fluid phase of the cylindrical core and diffusion and reaction in the outer reactive annulus. The proper normalization to be used in the definition of the generalized Thiele modulus as well as the aspect ratios that lead to the highest effectiveness are also determined. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 4020-4024, 2013