The North Atlantic Marine Boundary Layer Experiment (NAMBLEX), involving over 50 scientists from 12 institutions, took place at Mace Head, Ireland (53.32° N, 9.90° W), between 23 July and 4 September 2002. The aims of the campaign were to study the oxidation processes, atmospheric chemistry and composition of a number of species primarily in the Marine Boundary Layer. Campaign objectives were: to test quantitatively our basic understanding of oxidation processes in clean and moderately polluted air using observed correlations and comparisons of measured and model-predicted behaviour; to study extensively the chemistry of halogen species in the marine boundary layer through observation of reactive intermediates and their sources and sinks; to study the reactive nitrogen budget over the Atlantic Ocean; to examine the origins and role of reactive hydrocarbons in the MBL, and carbon budget reconciliation using comprehensive chromatography; and to investigate the size-distributed composition, internal mixing and Cloud Condensation Nucleus (CCN) activity of aerosols and the processes involved in new particle creation. A wide range of state-of-the-art instrumentation, developed using NERC and JIF funding, enabled detailed measurements of the boundary layer structure and atmospheric composition in the gas and aerosol phase to be made, providing one of the most comprehensive in situ studies of the marine boundary layer up to the time of the campaign. Measurements of some trace species were made for the first time during the campaign, which was characterised by predominantly clean air of marine origin, but more polluted air with higher levels of NOx originating from continental regions was also experienced. NAMBLEX was supported by measurements made in the vicinity of Mace Head using the NERC Dornier-228 aircraft and boundary layer wind profiles from the University of Wales, Aberystwyth mobile wind profiler. Participating institutions included the Universities of Leeds, York, East Anglia, Leicester, Birmingham, Bristol, Edinburgh, UMIST, Lancaster and Cambridge.