We report the syntheses, crystal structures, and the thermal properties of [Zn₃(HCOO)₆](CH₃OH)_1.5(H₂O)_0.5 (1_parent), [Zn₃(HCOO)₆] (2_empty), and six guest-inclusion compounds [Zn₃(HCOO)₆](I₂) (3_iodine), [Zn₃(HCOO)₆](C₄H₈O) (4_THF), [Zn₃(HCOO)₆](C₄H₄O) (5_furan), [Zn₃(HCOO)₆](C₆H₆) (6_benzene), [Zn₃(HCOO)₆](CH₃CN) (7_acetonitrile), and [Zn₃(HCOO)₆]((CH₃)₂CO) (8_acetone) as well as the H₂ and N₂ adsorption of 2_empty. The frameworks of all the compounds are similar, and consist of Zn-centred ZnZn₄ tetrahedral nodes of a distorted diamond structure. It is robust by virtue of the diamond structure and it displays permanent porosity in which the pores, occupying about 30% of the volume, can be reversibly emptied and refilled with solvents and gases without loss of crystallinity. The crystal-to-crystal transformation is assured by performing the guest-inclusion process by exposure of 2_empty to the vapours of the guests following the complete desolvation of 1_parent. Conformity is evidenced by the change in the lattice parameters proportional to the size of the guests. The framework is thermally stable up to 140°C at which it is transformed exothermically to a more compact form, β-Zn(HCOO)₂, that is stable to 270°C. Due to the flexibility and amphiphilic nature of the pores, consisting of both C–H and O arrays at the surface, 2_empty can take up a wide spectrum of both polar and non-polar guests of different size. The guests are confined in zig-zag molecular arrays within the channels where weak hydrogen bonds provide the main host–guest interaction. Except for acetonitrile, which sits in the central part of the channels, all the guests appear to line the wall of the channels.