The reaction of hydrogen peroxide with thiosulfate and sulfite in acidic solution is characterized by marked temporal pH variations suggesting autocatalytic nature of hydrogen ions. When carried out in a continuous-flow stirred tank reactor this reaction provides nonlinear dynamical regimes including periodic oscillations, chaotic behavior, and multiple steady states coexisting over a range of operating conditions. The aim of the presented experimental study is a classification of the role of species and the underlying mechanism in the periodic oscillatory mode by applying single pulse additions of chosen reaction species. The external perturbations at various phases of the periodically oscillating system may cause phase advance or phase delay of the oscillations. The resulting phase transition curves are obtained for hydrogen ions, hydroxide ions, thiosulfate ions, sulfite ions, and hydrogen sulfite ions. These curves are compared with the phase transition curves calculated using the prototype mechanisms representing categories of chemical oscillators established in previous work. We found our system to be compatible with the mechanism of the category 1CX.