A new pH oscillator has been constructed by combining the pH clock reaction H2O2-SO32--H+ with thiourea (Tu, (NH2)(2)CS) as a proton-consuming species. The system exhibited oligo-oscillatory behavior in a closed system, and large amplitude oscillations in a continuous-flow stirred tank reactor (CSTR) were observed in a narrow range of input concentrations, flow rate, and temperature. For the purpose of constructing the kinetic model, a reversed-phase ion-pair high-performance liquid chromatography (HPLC) and mass spectrometer (MS) were, used to track and determine intermediate species during the oxidation of thiourea by hydrogen peroxide. Experimental results illustrated that the four species: thiourea monoxide (TuO), formamidine disulfide (Tu(2)(2+)), thiourea dioxide (TuO(2)), and thiourea trioxide (TuO(3)) were formed during the oxidation process. A ten-step mechanistic model was proposed, where TuO was another key species participating in two proton feedback loops in addition to bisulfite. Numerical simulations based on this model agreed well with the experimental results.