Two case studies are presented of aircraft measurements (ozone, NOy, CO, and meteorological parameters) in the vicinity of fronts located over the eastern side of the North Atlantic Ocean during spring 1994. The aim of these studies was twofold: (1) to investigate whether frontal circulations can transport ozone from the boundary layer to the free troposphere in well-defined layer; and (2) to ascertain whether or not conveyor belts associated with extratropical cyclones exhibit well-defined chemical signatures. The first case study (March 2, 1994) sampled a well-defined ozone-enhanced layer within the free troposphere. It is demonstrated that this air was transferred from the boundary layer to the free troposphere during the development of a baroclinic wave. Two warm conveyor belts sampled within this flight (one associated with the developing baroclinic wave and the other with a mature low-pressure system) displayed clear and contrasting chemical signatures, a consequence of their geographically different origins. During the second case study (April 25, 1994), both the dry intrusion and the warm conveyor belt of a mature, occluded low-pressure system were sampled. Their chemical signatures (in particular, that of the dry intrusion) showed that interleaving of the two airstreams had occurred, probably in the vicinity of the occluded front. It is thus demonstrated that chemical measurements within conveyor belts provide valuable information on the nature, history, and extent of these coherent flows. ¿ 1998 American Geophysical Union