The synthesis of carbamates 3 and unsymmetrical alkyl carbonates 5 by reaction of diethyl carbonate with aliphatic amines or alcohols has been realized by using as heterogeneous catalyst a hybrid organic-inorganic material prepared by anchoring TBD to MCM-41 silica. Products are obtained in high yield and very good selectivity and the solid catalyst can be recovered simply by filtration and reused for different cycles without apparent lowering of activity. A supported N-carbethoxyguanidinium active intermediate is proposed, and some spectroscopic data are shown to support the mechanistic hypothesis.