The concentration of hydrogen ions is found to evolve periodically in the non-buffered chlorite-thiourea reaction. Period-doubling, homoclinic and quasiperiodic bifurcations have been uncovered in this study. Experiments further illustrate that under the reaction conditions employed in this investigation reducing the reaction temperature favors the occurrence of complex pH oscillations. These experimental results provide new insight into the underlying mechanism of the chlorite-thiourea reaction, in particular regarding the significance of the nonlinear oxidations of sulfur(-II) compounds. (C) 2004 Published by Elsevier B.V.