The WINDII instrument on board the UARS satellite made many thousands of measurements of the apparent brightness of the hydroxyl airglow as a function of tangent height during the night. The measurements were mainly at latitudes between 42 ¿S and 42 ¿N. Most of the profiles of volume emission rate indicate a simple layer with a peak between 85 and 90 km. However, from 5 to 25% of the measured profiles have a more complex structure for the layer profile, most frequently with two peaks separated by 4 to 6 km. The horizontal extent varies from 200 to several thousand kilometers, and the duration may be as long as 1 1/2 hours. Here the WINDII data from November 1991 to April 1995 are used to study the frequency of occurrence of the double-peaked layers as a function of latitude, season, and local time and to explore possible causes of the unusual profiles. Early in the night the double-peaked layers occur preferentially at low to medium latitudes, migrating toward the equator by midnight where they remain for the rest of the night. There is a pronounced asymmetry between the hemispheres which reverses between the March/April equinox and the September/October equinox. The solstice behavior is similar, though not so pronounced. Through simulation three possible causes to the observed OH* double-peaked layers were identified. Gravity waves are a likely source of isolated small-scale events but the large horizontal scale, long-duration structures are more likely to result from temperature inversion layers or mixing events or a combination of the two. ¿ 2000 American Geophysical Union