Humidity-induced multiple-step single-crystal to single-crystal (SC-SC) transformations are observed in the cobalt phosphonate (NH4)(3)[Co-2(bamdpH)(2)(HCOO)(H2O)(2)] (1), where bamdpH(4) is (benzylazanediyl)bis(methylene)diphosphonic acid, [C6H5CH2N(CH2PO3H2)(2)]. Under high-humidity conditions (95% RH), compound 1 experiences hydrolysis at 60 degrees C which is accompanied by the transformation from a double-chain structure of compound 1 into a single-chain structure of [Co(bamdpH(2))(H2O)(2)]center dot 2H(2)O (2). When the humidity is below 10% RH, part of the lattice water in compound 2 can be released, forming a third phase, [Co(bamdpH(2))(H2O)(2)]center dot H2O (3). The structural transformation processes have been monitored by infrared and proton conductivity measurements.