Three ethylene adducts to CuAlCl4 have been characterized by single crystal and/or powder X-ray diffraction, C-13, Al-27 and Cu-63 MAS NMR and diffuse reflectance UV-vis spectroscopy. (C2H4)(2)CuAlCl4, a = 7.1274(5) b = 12.509(1) c = 11.997(3) beta = 91.19degrees, Pc, Z = 4; alpha-(C2H4)CuAlCl4, a = 7.041(3) b = 10.754(8) c = 11.742(9) beta = 102.48(6), P2(1), Z = 4 and beta-(C2H4)CuAlCl4, a = 7.306(2), b = 16.133(3), c = 7.094(1), Pna2(1), Z = 4. Up to 2 equiv of ethylene (similar to200 cm(3)/g relative to stp) are sorbed at room temperatures and pressures as low as 300 Torr. The ethylene ligands are bound to copper (1) primarily through a sigma-interaction, because the AlCl4- groups also bound to copper prevent any significant pi-backbonding. The olefin binding is reversible and has been characterized by gravimetric and volumetric adsorption analysis and by time and pressure resolved synchrotron powder X-ray diffraction. Comparison of the parent crystal structure to those of the adduct phases provide an atomistic picture of the sorptive reconstruction reactions. These are proposed to proceed by a classic substitution mechanism that is directed by the van der Waals channels of the parent crystalline lattice.