The emissions test capability developed at New York's Automotive Emissions Laboratory is described as applied to realtime measurements from in-use three-way-catalyst vehicles. Realtime data from tailpipe emissions analyzers, engine air/fuel ratio analyzer, and vehicle speed/acceleration/power from an electric chassis dynamometer are polled several times per second by a minicomputer data acquisition system. These data are obtained in parallel with conventional CVS bag results. Examples of this capability are demonstrated with test results from two in-use computer-controlled catalyst cars. Results are shown as applied to modal analysis and inter-relationships among collected data.State air pollution control program areas where this capability can be applied are suggested. Improvements and refinements are discussed in preparation for these tasks.