Nanoporous thermostable polycyanurate (PCN) films were generated by in-situ metal complex-catalyzed polycyclotrimerization of 4,4′-ethylidenediphenyl dicyanate in the presence of 20–50 wt.% of inert high-boiling temperature liquids as porogens, i.e. dioctyl, dibutyl or dimethyl phthalates (DOP, DBP or DMP), followed by their extraction from the densely crosslinked PCN frameworks, drying, and additional annealing at various temperatures. The nanoporous structure for all PCN-based films developed was investigated by SEM, BET, and DSC-based thermoporometry. Furthermore, the thermal and dielectric properties of the nanoporous films were also examined. The method developed allows producing nanoporous films for membranes with controlled thermal and dielectric properties as well as porosity parameters.