The thermal stability of an organic zeolite material, namely 2,4,6-tris(4-bromo-3,5-difluorphenoxy)-1,3,5-triazin (Br-3,5-DFPOT), was improved by fluorination of 2,4,6-tris(4-bromophenoxy)-1,3,5-triazin (BrPOT). The open pore structure (van der Waals diameter of 10.5 Å) of the modified zeolite was observed up to 110 °C in comparison to 70 °C for BrPOT. Nitrogen sorption at low temperature showed a type I isotherm and derived pore volumes thereof are in agreement with structural data. It was observed here that Br-3,5-DFPOT crystals preserving the open pore structure could only be obtained below a typical size of about 50 μm. The improved thermal stability of the fluorinated system is attributed to an enhancement of the strength of the Br3-synthon.