ABSTRAC: This study describes the first demonstration of using monocyclic C18 and B9N9 as active materials for the sensing and removing of chemical warfare agents (CWAs) such as formaldehyde (Fd), phosgene (Ph), and thiophosgene (TPh). The sensitivities and adsorption capacities of both C18 and B9N9 analogues toward considered molecules are thoroughly characterized by optimized geometries, energetic stabilities, electronic behaviour, and optical properties at the M05-2X-D3/6-31++G(d,p) method of density functional theory (DFT). In the complexes, the analytes interact through the active cavity of the monocyclic (ring) systems. It is found from the BSSE corrected nergies that the adsorption of analytes on B9N9 is energetically more favorable than that on C18. Based on the dsorption energies, the sensitivity of surfaces toward formaldehyde and phosgene is higher than the thiophosgene molecule. The adsorption energies of Fd@B9N9, Ph@B9N9 and Thp@B9N9 complexes are − 7.47, − 7.45, and − 4.96 kcal mol− 1, respectively which is about 2.19, 1.56, and 0.69 kcal mol− 1 exothermic than Fd@C18, Ph@C18, and Thp@C18 complexes, respectively. Symmetry adapted perturbation theory (SAPT0), reduced density gradient (RDG) and quantum theory of atoms in molecule (QTAIM) results illustrate that dispersion forces play an important role in stabilizing these complexes. The variations of the electronic properties are explained based on HOMO LUMO orbitals, the density of state (DOS), NBO charge transfer, and UV-Vis analysis. The significant variations in the electronic parameters reveal that CWAs can bind more strongly with these ring systems which is more pronounced in B9N9. Therefore, these monocyclic systems may be considered a potential candidate for sensor application.