We designed, prepared, and characterized three cup-shaped cavitands 1-3 for trapping organophosphonates (O=PR(OR')(2), 118-197 A3) whose shape and size correspond to G-type chemical warfare agents (132-186 A(3)). With the assistance of computational (molecular dynamics) and experimental CH NMR spectroscopy) methods, we found that host [1-H-3](3+) orients its protonated histamine residues at the rim outside the cavity, in bulk water. In this unfolded form, the cavitand traps a series of organophosphonates 5-13 (K-app = 87 +/- 1 to 321 +/- 6 M-1 at 298.0 K), thereby placing the P-CH3 functional group in the inner space of the host. A comparison of experimental and computed H-1 NMR chemical shifts of both hosts and guests allowed us to derive structure activity relationships and deduce that, upon the complexation, the more sizable P OR functional groups in guests drive organophosphonates to the northern portion of the basket [1-H-3](3+). This, in turn, causes a displacement of the guest's P-CH3 group and a contraction of the cupshaped scaffold. The proposed induced-fit model of the recognition is important for turning these modular hosts into useful receptors capable of a selective detection/degradation of organophosphorus nerve agents.