Measurements of a suite of atmospheric trace constituents made from the NASA DC-8 aircraft, while it was making vertical profiles during the Pacific Exploratory Mission A (PEM-West A) to the western Pacific in September--October 1991 have revealed a layered structure in much of the region. Ozone, water vapor, carbon monoxide, and methane were available continuously and are the primary constituents used to define the layers; nonmethane hydrocarbons, carbon dioxide, hydrogen peroxide and methyl-hydroperoxide were available less frequently but were also used. From 105 vertical profiles, sampled at vertical speeds of about 6 m s-1, over 500 layers were identified using O3 and H2O; their mean thickness was about 400 m. When these layers could be identified from lidar along-track cross sections of O3 made from the aircraft, they were found to extend over distances of 100--200 km or more. Combinations of several constituent deviations from vertical running means were used to identify possible layer sources; examples are high O3, low H2O, CO, and CH4, corresponding to the extrusion of stratospheric air into the troposphere; low O3 and high H2O, originating from convection from the tropical boundary layer; high O3, CO and CH4, low H2O, coming from convection from the continental boundary layer and transport in the upper troposphere, sometimes from distant sources. The layers show no clear relationship with wind velocity profiles measured simultaneously on the aircraft; there is some relationship with temperature and potential temperature, with some layers which show a stratospheric signature lying on isentropes that are in the lower stratosphere at higher latitudes. Potential vorticity (PV) is used as an additional tracer to check the conclusions about the different types of layers. While PV is often used as an indicator of stratospheric air, its use as an indicator of convection is also illustrated here. ¿ American Geophysical Union 1996