Recent developments in the energy markets, and the surge and dip in crude oil prices over the last few years, have renewed the interest in the workings of the two main price setting markets: London's International Petroleum Exchange (IPE) and New York's Mercantile Exchange (NYMEX). The interaction of these two markets, when both of them are open (synchronous trading) and when only London is open (asynchronous trading), is important, in view of the fact that most participants take positions in both markets.This paper looks at how London is affected by New York by analysing the transaction duration of the IPE Brent futures contract, both when the NYMEX WTI futures contract is being traded and when NYMEX is closed. Using tick-by-tick data obtained from IPE, transaction durations are found to form two distinctive and inverted U-shaped patterns.Autoregressive conditional duration (ACD) model, first introduced by Engle and Russell, is applied to the data. Parameters of IPE morning and afternoon are significantly different from each other, underlining the dominant effects of NYMEX on IPE trading. The results from the current analysis reinforce previous results by the authors, which indicate that NYMEX is a leading price setter in crude oil futures prices and has a dominant effect on the IPE-traded contracts. (C) 2002 Elsevier Ltd. All rights reserved.