Four catalysts, nominally identical except for Pt/Rh ratio, were tested for emissions of regulated pollutants and for HCN, NH3, SO4(-2) and H2S. Fuel economy was also measured. The tests were conducted over the FTP, CFDS, and 64 and 80 kph cruise modes, using three simulated malfunction modes: O2 sensor disconnected-idle adjustment correct; O2 sensor disconnected-idle adjustment enriched; 13% misfire. The catalysts were prepared by Engelhard Industries under conditions similar to those used in making commercial three-way catalysts. A prototype Volvo TWC vehicle was the test car. The four catalysts contained respectively: Rh only, 17% Rh-Pt; 5% Rh-Pt; Pt only. Tests were also run with no catalyst under the malfunction modes and with each catalyst under normal engine conditions. Of the fifteen possible test combinations (four catalysts and no catalyst x 3 malfunction modes), twelve were chosen to be run, with four replicates, giving sixteen runs total. No HCN was detected at the tailpipe under normal operating conditions over the FTP. However, under the malfunction modes, HCN emissions were generally found. During the FTP and CFDS cycles all of the catalysts appeared to remove some HCN from the exhaust gas under all malfunction modes, but during cruise some catalysts under certain malfunctions caused a further increase in tailpipe HCN emissions. The maximum level observed was about 40 mg/km.