The adsorption behavior and mechanism of Reactive Blue 19 (RB19) on cucurbit[6]uril (CB[6]) and cucurbit[8]uril (CB[8]) were investigated. The adsorption isotherm data obtained at different temperatures were fitted well to the Langmuir isotherm, and according to this model, CB[8] and CB[6] exhibited maximum monolayer adsorption capacities of 714.29 and 100.5 mg/g, respectively, at 298.15 K. The adsorption thermodynamic functions Delta G, Delta H, and Delta S were evaluated and revealed that RB19 adsorption onto CB[8] and CB[6] is a spontaneous and enthalpy-driven process. The adsorption process was determined to follow pseudo-second order kinetics, indicating that chemisorption dominates the adsorption process. Fourier tranform IR spectroscopy, thermogravimetric analysis, and density functional theory (DFT) calculations revealed that the formation of an inclusion complex is the main driving force of adsorption. The phenyl and sulfone moieties of RB19 reside inside the cavity of CB[8], but because of the small cavity, only the sulfone of RB19 resides inside the cavity of CB[6]. Time-dependent DFT calculations revealed that all of the absorption bands of RB19 derive from pi -> pi* transitions, while for the adsorption product of CB[8], the bands located at 590 and 287 nm derive from pi -> pi* transitions and the bands located at 254 and 202 nm mainly derive from intermolecular charge transfer (ICT).