Vapor-responsive magnetic materials are highly promising for applications as chemical switches or sensors. Compared with porous materials, nonporous species benefit in overcoming the intrinsic conflict between magnetic exchange and porosity but usually suffer from the powdering of single crystals, which hinders the understanding of the structural nature of vapor response and magnetic switch. Single-crystal-to-single-crystal (SCSC) transformation of nonporous compounds through the desorption/absorption of gaseous HCl is unprecedented. Reported here is a discrete nonporous copper (II) complex, (H3O) [K(15-crown-5)(2)] [CuCl4], that exhibits reversible SCSC transformation and magnetic change by the chemisorption/desorption of HCl and H2O. Significant changes in the coordination number (4 <-> 3), space group (P (1) over bar <-> P2(1)/c), color (green <-> red), and magnetic behavior (antiferromagnetic <-> paramagnetic) were found during the SCSC transformation.