In situ measurements of N2O and O3 aboard the Observations from the Middle Stratosphere (OMS) balloon payload in the tropics are presented. In November 1997 at 7 ¿S very distinct laminated structures in the N2O profile were observed in contrast to earlier studies based solely on O3 profiles. The laminae probably are an observation of Rossby wave-breaking events that lead to transport of mid-latitude air into the tropics. Using a photochemical model of the ozone response within those air parcels, their residence time in the tropics is estimated to be 4 to 6 weeks. In February 1997 at 7 ¿S a similar vertical profile of N2O revealed no mid-latitude laminae. We present dynamical arguments to support the existence of such laminae in November, but not in February. ¿ 1998 American Geophysical Union