Investigations of the kinetics of molecular transfer across the liquid/gas interface and the effect of a molecular monolayer are of considerable interest as a model for certain biological and environmental processes. In this work, a combined scanning electrochemical microscopy (SECM)-Langmuir trough technique has been used to investigate the effect of the chemical character and mechanical compression of molecular monolayers on the rate of oxygen transfer across the air/water (A/W) interface. Specifically, monolayers comprising the fatty alcohol 1-octadecanol and the phospholipid L-alpha-dipalmitoyl phosphatidic acid were considered. A mercury hemispherical microelectrode probe has been used to measure interfacial kinetics in SECM, and a numerical model has been developed for mass transport in this configuration to allow quantitative analysis of experimental data. The results obtained suggest that, for both monolayers, the oxygen-transfer rate across the interface decreased compared to that across the clean interface, with the blocking effect becoming more pronounced as the surface pressure of the monolayer increased. A simple energy-barrier model was used successfully to interpret the dependence of the rate constant of oxygen transfer on the surface pressure. The experimental data also provide evidence for the effect of the SECM probe on the deformation of the water surface at very close distances to the A/W interface.