The results and interpretation of trajectory analyses for the Pacific Exploratory Mission-West A (PEM-West A) experiment are described. The calculations are based on gridded meteorological fields; the first step is the preparation of an analysis of winds and other variables on selected isentropic surfaces. The sources of uncertainty and error in this and the subsequent step of trajectory analysis are described. The most important of these are limitations in the adequacy of the adiabatic approximation and flaws in the parent meteorological fields, particularly caused by data voids over open ocean areas. Examples are presented of the trajectory results for points along the flight track of the DC-8 during the September--October 1991 expedition. The corroboration of air mass classifications based on trajectories by chemical characterization based on the in situ and remotely sensed observations from the aircraft discussed elsewhere in this issue is noted. An analysis of multiyear patterns of airflow in the western Pacific area, based on trajectory calculations for the September--October period for 1988--1990, is also presented. ¿ American Geophysical Union 1996