We synthesized cavitands containing COOH moieties at the upper rim of the cavity and evaluated the interaction between these cavitands and the sarin nerve gas simulant, dimethylmethylphosphonate (DMMP) using surface plasmon resonance (SPR) spectroscopy. The carboxylic acid group on the cavitand is expected to form a hydrogen bond with the PO group of the organophosphorus vapors. Films of these cavitands produced a rapid and reversible SPR response to low concentrations of DMMP. We observed concentration dependent sorption of the DMMP molecule into the COOH containing layer in the ppb to ppm range. Spin-cast films and Langmuir–Blodgett (LB) depositions of bilayer thick cavitand films produced identical SPR shifts upon exposure to DMMP. The sensitivity of the sensor was enhanced via LB deposition of multiple bilayers. Eight-layer-thick films of the COOH cavitand showed sensitivity to DMMP concentrations as low as 16 ppb. The orientation of the COOH group into or out of the cavity did not affect DMMP binding, but strongly influenced the water uptake. In both cases the molecular recognition event responsible for the DMMP uptake has been elucidated via crystal structure analyses of the complexes of COOH in/out cavitands with DMMP. Furthermore, we demonstrated that the COOH containing cavitand had an SPR sensitivity to DMMP higher than the standard fluoropolyol sensing layer, and that the cavitand layer was less prone to water vapor and alcohol interferences. Hence, cavitand layers containing a COOH moiety are promising for use as sensitive and specific sensors for nerve gas agents.