A pair of CuII coordination isomers [Cu(pmt)2] · 4H2O (1) and {[Cu(pmt)2] · 2H2O} n (2) [Hpmt = 2-(2-pyridylmethylamino) ethanesulfonic acid] have been prepared by reaction of the same proportion CuCl2 · 2H2O and Hpmt but with different experimental methods in water–methanol mixed solution, and were characterized by X-ray diffraction, elemental analysis, IR spectrum. X-ray analysis indicates that complex (1) is a mononuclear complex in which two deprotonated pmt ligands coordinate in a facial tridentate arrangement about the CuII atom. While complex (2) is a coordination polymer, in which four N atoms of pmt ligands coordinate to one CuII atom but its sulfonate O atoms bond to adjacent CuII atom. Thus 12-membered rings (–Cu–N–C–C–S–O–)2 are obtained between two neighbouring CuII atoms and they connect one another to form this new polymer. Crystal data: [Cu(pmt)2] · 4H2O (1), Mr = 566.10, monoclinic, P2 1/c, a = 9.1950(8) A, b = 11.5093(10) A, c = 11.2304(10) A, β = 105.5550(10)°, Z = 2, V = 1144.96(17) A3, R 1 = 0.0407, wR 2 = 0.1242 [I > 2σ (I)]; {[Cu(pmt)2] · 2H2O} n (2), Mr = 530.07, triclinic, P-1, a = 7.6165(19) A, b = 8.806(2) A, c = 9.592(4) A, α = 104.933(4)°, β = 106.732(4)°, γ = 109.503(3)°, Z = 1, V = 534.2(3) A3, R 1 = 0.0470, wR 2 = 0.1082 [I > 2σ(I)]. A pair of CuII coordination isomers [Cu(pmt)2] · 4H2O (1) and {[Cu(pmt)2] · 2H2O} n (2) [Hpmt = 2-(2-pyridylmethylamino) ethanesulfonic acid] have been prepared by reaction of the same proportion CuCl2 · 2H2O and Hpmt but with different experimental methods in water–methanol mixed solution.