The reuse of MgO byproducts as SO2 absorbents in a sustainable closed-loop process follows the guidelines of legislation and economic optimization. The aim of the present study was to enhance the desulfurization performance of an MgO byproduct by sieving to different size fractions and relating them with their reactivity and physical characteristics. The byproduct presented a rather irregular size distribution, with Mg presented as MgO, Mg(OH)(2), and MgCO3 and Ca as CaO, Ca(OH)(2), and CaSO4. The time of saturation (t(s)) was used for evaluating the desulfurization performance of each fraction with 100% removal efficiency. Thus, two different conditions for enhancing the desulfurization performance were described. A chemically established condition where the sorption capacity is improved by adding more solids and a second one dependent only on physical parameters. Accordingly, sieving to the finest size fraction could improve the desulfurization capacity close to an optimum value (2.9 kg of solids can totally neutralize 1 m(3) of SO2). Taking into account that the average grinding cost to a particle size below 0.075 mm is 11.54 euro per ton, adding an extra operational unit prior to desulfurization might be a feasible alternative for attaining 100% removal efficiency. Moreover, CaSO3 and CaCO3 were the main reaction products from desulfurization, which could be reused as construction aggregates. The procedure for finding a valuable methodology for improving the SO2 removal capacity of this kind of byproduct could be extended to other wastes and residues.