The fate of atmospheric aerosol is currently attracting increased attention from the scientific community because its impact on the Earth's radiative balance and on cloud formation is still largely unknown. To understand the ageing process of aerosol it is helpful to investigate heterogeneous reactions occurring between organic surface films and gas-phase oxidants. While most studies have focused on the abundant daytime oxidants 0 3 and OH, during the night-time the OH concentration is very low and the concentration of the more potent NO:! becomes significant. In this thesis insoluble organic monolayers at a planar air-water interface were used as proxies for films on atmospheric aerosol, and the surface excess kinetics were monitored in situ using a combination of neutron reflectometry (NIt) and ellipsometry. A range of compounds were chosen to allow a study of the effects on the reaction kinetics and product formation of the chain length, type of headgroup and degree of unsaturation on the organic molecule as well as the type of oxidant. To allow the work to be performed several key developments were carried out: a refined method for NR data analysis, the commissioning of a dedicated new miniature sample chamber, rigorous calibration of the oxidant concentrations and development of a kinetic model. The limitations of ellipsometry as a substitute for NIt for the study of pure syst.ems were explored, yet its potential to provide complementary information about product formation was demonstrated. Six reactions of pure organic monolayers revealed that the degree of unsaturation of the chain was by far the biggest factor for the reaction rate, although other differences were also systematically examined. Two binary mixtures were investigated showing that the reaction rate is consistently lower in mixed films, which may help to explain discrepancies in the literature between laboratory studies and field measurements.