This study examines whether changes in the frequency of market clearing or changes in trading hours on competing exchanges that use different auction systems affect the volatility of futures prices. In particular, this study exploits a natural experiment in the frequency of market clearing of stock index futures contracts traded on the Taiwan Futures Exchange (TAIFEX) to assess whether successive increases in the frequency of market clearing are associated with changes in the volatility of futures prices. The impact of changes in the trading hours on the TAIFEX and on the competing Singapore Exchange (SGX) where a similar Taiwanese stock index futures contract trades under a continuous auction market regime is also examined. The evidence for the impact of an increase in the frequency of market clearing on volatility is mixed. However, the introduction of simultaneous opening times for the TAIFEX (which batches orders at the open) and the SGX (which does not) is associated with a significant reduction in the volatility in SGX Taiwanese stock index futures prices. (c) 2007 Wiley Periodicals, Inc.