The formation of the active phases during the activation process of monolithic catalysts based on V2O5-K2SO4 supported on diatomaceous earth for SO2 to SO3 oxidation in flue gases, has been shown to be a crucial factor to achieve satisfactory catalytic performance. As the temperature is increased from room temperature to 470 degrees C, SO2 and SO3 are taken up by the green catalyst and the precursors are transformed into the active species. The role of each component of the catalyst during the activation was analyzed by studying the behavior towards SO2 adsorption of four materials, which contained: diatomaceous earth, diatomaceous earth + V, diatomaceous earth + K, and diatomaceous earth + V + K. The influence of the potassium sulfate accessibility in the green catalyst was studied by using two different preparation methods, which gave rise to differences in the catalysts SO2 adsorption properties and catalytic performance. Furthermore, the influence of the activation atmosphere was studied using nitrogen, oxygen or a flue gas composition. It was shown that pyrosulfate species should be formed at temperatures below 400 degrees C, to keep the vanadium in the active 5+ oxidation state.