The hydrothermal syntheses, crystal structures and magnetic behavior of two compositionally different piperazine pillared copper molybdates, [{Cu(pip) 0.5 }MoO 4 ] (1) and a hitherto unknown [Cu(pip)MoO 4 ] (pip=piperazine) (2), are reported. Both 1 and 2 exhibit three-dimensional covalent frameworks constructed from bimetallic oxide layers pillared by piperazine; however, the {CuMoO 4 } networks are quite distinct. The Cu–Mo–O layers in 1 are made of edge-shared {CuO 4 N} square pyramidal pairs linked through {MoO 4 } tetrahedra, in contrast to the sheets in 2 that are built of corner-sharing {MoO 4 } tetrahedra and {CuO 3 N 2 } square pyramids. Self assembly of the two pseudopolymorphs, 1 and 2, is interpreted in terms of molecular recognition between reasonable soluble molecular species in the supramolecular reaction along the mechanistic approach proposed by Ramanan and Whittingham for rationalizing metal-organic framework structures. Crystal data: 1, Triclinic, space group P-1, a=5.5765(8)Å, b=6.8304(10)Å, c=9.2379(14)Å, α=100.688(2)°, β=101.462(2)°, γ=112.624(2)°, Z=2; 2, Orthorhombic, space group Pbca, a=11.3899(11)Å, b=10.7726(10)Å, c=13.2541(12)Å, Z=8.