[M(Py){Ni(CN)(4)}] (M = Fe, Co, Ni) frameworks were synthesized by solution method and their hydrogen storage properties were measured using a PCT measuring system. The series of frameworks were found to adsorb twice hydrogen predicted by monolayer adsorption model. Compared with the hydrogen adsorption property of FeFe(CN)(6), this strange phenomenon is elucidated at a molecular level. The pore diameter of Py series is just enough to contain two hydrogen molecules but only one nitrogen molecule is permitted to get in, which leads to the apparent double-layer adsorption. It is concluded that pores with free diameter smaller than 0.42 nm are not favorable for hydrogen storage because no more than one hydrogen molecule can get into them. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.