The formaldehyde-sulfite reaction is an acid to alkali clock reaction in batch, which displays complex behaviour in a flow reactor. Current published mechanisms do not account for the behaviour in all open system. In this Letter, we construct a minimal model based on this system and demonstrate that a base-catalysed rate-determining step coupled with an appropriate OH- consuming reaction can result in bistability and oscillations. The model may provide a method for the design of organic-based pH oscillators. (c) 2005 Elsevier B.V. All rights reserved.