A photochemical trajectory model together with a master chemical mechanism and a highly speciated emission inventory have been used to characterise the multi-day formation of photochemical ozone in north west Europe for 123 organic compounds. Multi-day reactivity is apparent for both short-lived as well as long-lived organic compounds and arises for a number of mechanistic reasons. Unreacted organic compounds may be carried over from one day to the next. Degradation products formed on the first day may take part in the formation of ozone over longer timescales compared with those of ozone formation from parent compounds. For the highly reactive alkenes and carbonyls, two additional mechanisms account for much of the ozone formation on the second day. These involved the formation of peroxyacylnitrates and aldehydes during the first day which were carried over into the second day to stimulate additional ozone formation. The implications of these findings are discussed for the highly simplified and parameterised chemical mechanisms employed in some policy applications. (C) 2004 Elsevier Ltd. All rights reserved.