We have analysed the use of chemically reactive liquid curtains for the containment of accidental releases of phosgene. Experiments performed on absorbers with widely different diffusion times gave results which appeared to imply a different stoichiometry of the main reaction between phosgene and sodium hydroxide. The difference has been explained with a model of chemical absorption : when the diffusion time is high, direct reaction of phosgene with water in a very thin layer near the interface results in the formation and desorption of carbon dioxide. This effect seems to have never been previously recognized as a possible one in chemical absorption.