Gaseous molecular iodine (I 2 ) is an important source of reactive iodine in the marine atmosphere, but the sources of I 2 are not well understood due to the lack of an easily accessible, sensitive and robust technique for analysis. In this study a microfluidic derivatisation technique combined with GC–MS has been developed to measure gaseous I 2 . Good linearity in the range of 0.2–416ppb and low detection limits varying from 6 to 25ppt for different derivatisation reagents have been achieved, which is a substantial improvement in sensitivity compared with the spectrophotometric method (detection limit of 1.20ppb) in our previous study [L.J. Carpenter, S.M. MacDonald, M.D. Shaw, R. Kumar, R.W. Saunders, R. Parthipan, J. Wilson, J.M.C. Plane, Nature Geoscience, 6 (2013) 108–111]. The microfluidic technique was employed to quantify I 2 produced from the heterogeneous reactions of potassium iodide solution and ozone. Good agreement was observed between the results of the microfluidic technique and the simulation of a coupled surface water–air kinetic model in the amount of I 2 produced on the ozonolysis of iodide solutions.