Laser-ablated magnesium species were codeposited with SO2 in excess argon or neon on the substrate at 4 K. The reactions mainly produced Mg(eta(2)-O2S), Mg(eta(2)-O2S)(2), Mg-2(eta(2)-O2S), OMg2(eta(2)-SO), and Mg(eta(2)-SO) complexes, which were identified by isotopic substitutions and density functional frequency calculations (B3LYP and BPW91). In addition, the collected infrared spectra suggest that the single Mg atoms could react with SO2 to form the Mg(eta(2)-O2S) complex on annealing, which further reacts with SO2 to produce the Mg(eta(2)-O2S)(2) complex on irradiation. In contrast, the reactions of magnesium dimers lead to cleavage of the S-O bond in SO2 on irradiating. Structural and bonding characteristics of these generated complexes, which shed light on the different performances of single Mg atom and its dimer in their reactions with small molecules, are discussed.