The potential importance of iodine for marine boundary layer (MBL) chemistry has found increasing recognition in recent years. However, observations of the key iodine species are sparse and the chemical reactions of the iodine oxides are not well understood. Here we present Differential Optical Absorption Spectroscopy observations of IO, OIO in the MBL of the Gulf of Maine, U.S., during Summer 2004. We report the first daytime observation of OIO, indicating that this compound is rather photostable. Mixing ratios of IO were similar to, and those of OIO higher than, values reported for European coastal sites. Calculations with the one‐dimensional model MISTRA show that the observed simultaneous presence of elevated OIO and NOx cannot be explained by currently known iodine chemistry. Our results lead to the conclusion that thus far unknown chemical reactions of iodine oxides, probably involving iodine nitrates, might occur in the MBL.