The thermodynamics and kinetics of the reaction of cucurbituril (Cuc) with 4-amino-4′-nitroazobenzene (DO3) and 4,4′-diaminoazobenzene (Diam) have been studied in aqueous HCl, and it is shown that DO3 is well suited as an indicator for uncomplexed Cuc (actual concentration of Cuc) and Diam is well suited as an indicator for the total concentration of Cuc (sequestering agent) in aqueous solutions. The formation of 1∶1 guest–host complexes is observed. For proton concentrations up to 1 mol dm^–3, DO3 reacts according to eqn. (a), with the values for the dissociation constant of DO3H⁺ log(K_a/dm³ mol^–1) = –2.42, DO3H₂²⁺ log(K_b/dm³ mol^–1) = 0.53, for the stability constant of CucDO3H⁺ log (K_c/mol dm³) = 8.05 and for the rate constant of complex formation log (k_f/dm³ mol^–1 s^–1) = 5.25. Reaction enthalpies and activation energies have also been determined. The complexation of Cuc with Diam has been studied in 0.1–1 mol dm^–3 HCl where a very strong complexation has been observed.