The metal-organic framework [Cd15(5MT)18(OH)4(SO4)4] (5-MT = 5-methyl-1H-tetrazole) with an unprecedented Cd(3)(micro3-OH)(micro3-SO4) core and two kinds of tetrazole coordination modes was formed by self-assembly and in situ synthesis of the 5-MT tetrazole ligand; the structure exhibits an unusual 3(9) x 4(12) x 5(15) topological network and shows reversible shrinkage and expansion in a crystal-to-crystal dehydration/rehydration process.