Hyper-cross-linked polycyanurate networks (CE-P1 and CE-P2) were synthesized by means of thermal self-cyclotrimerization from two triangular cyanate resin monomers 1,3,5-tri(4-cyanatophenyl)benzene and 1,3,5-tricyanatobenzene, respectively. Interestingly, it was found that CE-P1 exhibited microporous characteristics and a moderately large BET surface area. The two narrow peaks in the nonlocal density functional theory (NLDFT) curve appeared at 0.57 and 1.01 nm. In contrast, the CE-P2 sample had a small surface area and broad pore-size distribution with major pores of around 3.39 nm, which indicated a mesoporous material. The reason for this was interpreted in terms of the geometric configuration, steric hindrance, and reactivity of the cyanate monomers. The adsorptions of CO2, H2, benzene, n-hexane, and water vapors were investigated by correlating the data with the porosity parameters, chemical structure, and composition of the two networks. The results showed that the vastly distinct pore properties significantly influenced the adsorptions of gases and vapors. In particular, organic vapors such as benzene and n-hexane tended to be adsorbed on the pore surface owing to their affinity and thereby the adsorption amounts were tightly attached to the surface area of the samples. On the contrary, the hydrophobic nature of polymers made the water molecules preferentially condense within the pores so that the pore size rather than the surface area became the dominant factor influencing the adsorption of water vapor.