Experiments have been conducted in a Knudsen cell flow reactor in order to investigate the heterogeneous interaction of HOI on water ice and sulfuric acid at temperatures relevant to the lower stratosphere. They reveal a nonreactive uptake on ice whose rate is temperature independent in the range 180--200 K, whereas a negative temperature dependence has been observed in the range 195--250 K on liquid sulfuric acid. Two processes take place at the interface, namely, decomposition of HOI resulting in the formation of I2 and partial loss of HOI in the condensed phase without the apparent formation of a product. This latter fraction of HOI could not be retrieved by conversion to ICl or IBr upon addition of HCl or HBr. The heterogeneous reactions of HOI with alkali salts at ambient temperature have also been studied. HOI is found to interact reactively with KBr with an uptake coefficient &ggr;=(6¿2)¿10-2. I2 is observed as the main product, but IBr has also been detected at a branching ratio of approximately 6:1. Implications for atmospheric chemistry are briefly discussed. ¿ 1999 American Geophysical Union