CO2 is almost vertically adsorbed and desorbed at 0.0106-0.0073 relative pressure at 273 K, respectively, on the preevacuated Cu-complex. crystals which have no open micropores from the crystallographic structures. These vertical adsorption and desorption steps are named gate effects. Here, guest water molecules are evolved during the pre-evacuation at 373 K and the crystal structure with X-ray diffraction examination does not change due to pre-evacuation. The X-ray diffraction patterns also show no change before and after the steps. The adsorption/desorption gate effects and the remarkable hysteresis are explained in terms of the molecular valve model stemming from the reorientation of the bending vibration of the hydrogen bonding. This model can explain the observed hysteresis in the gate effect; the spring constant difference of the molecular valve for desorption and adsorption is estimated to be a similar order to that of the hydrogen bond in bulk ice. (C) 2002 Elsevier Science B.V All rights reserved.