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Sulfonation is a versatile method for the production of acidic materials, e. g. for applications in catalysis. In this study different sulfonation techniques for the production of acidic ordered mesoporous materials are investigated. SBA-15-like mesoporous organosilanes bearing aromatic moieties are sulfonated in the liquid phase with oleum as well as in the gas phase under significantly milder conditions with sulfur trioxide. Functionalization via liquid phase sulfonation at elevated temperatures proceeds at the expense of partial or complete collapse of the pore system. In contrast, gas phase sulfonation with sulfur trioxide at ambient temperature allows for similar sulfonation yields, while maintaining the textural properties due to the milder reaction conditions. Overall, acid functionalization of mesoporous organosilica using a gas phase sulfonation route is reported for the first time, enabling both high acid capacities and preservation of the pore structure.