This letter describes an experiment that examines the ability of the University of Wisconsin (UW) hybrid isentropic-sigma (&thgr;--&sgr;) and sigma (&sgr;) coordinate models and the NCAR Community Climate Model 2 (CCM2) to transport and conserve the joint distributions of isentropic potential vorticity (P&thgr;) and a source-free inert trace constituent related to the initial distribution of P&thgr;, called proxy ozone (O3), during 10-day isentropic integrations. Under the idealized conditions of this experiment, the governing equations for the atmospheric continuum require that the initial joint distribution of P&thgr; and O3 be conserved, thereby establishing a test of model accuracy from statistical comparisons of paired values of O3 and P&thgr;. Any decrease in the initial correlation of unity of P&thgr; and O3 after integration is an objective measure of a model's skill. Results show that correlation coefficients for the UW &thgr;--&sgr; model remain higher than those from CCM2 and the UW &sgr; model, demonstrating an inherent advantage in the simulation of trace constituent transport relative to dynamical processes in isentropic versus sigma coordinate models. ¿ American Geophysical Union 1996