On occasions automotive fuels have been contaminated by adventitious admixtures of silicon (Si)-containing compounds which have deleterious effects on automotive catalysts and oxygen sensors. The deactivation of monolithic automotive catalysts by fuel-derived silicon is due to deposition of crystalline silica (∝-SiO2) on the catalyst surface which causes mass transfer limitations and may ultimately result in plugging of the monolith. Stoichiometric conversions efficiency of three-way catalysts (TWCs) from various low-mileage vehicles were significantly deteriorated; e.g., from typical three-way efficiencies of −95% conversion to <50% conversion at 550°C after only 1500 mi of vehicle use. Laboratory aging of a TWC exposed to combustion products of isooctane fuel containing 20 ppm Si resulted in a continual decline in three-way conversions to <40% after 15,000 simulated miles. Catalyst activity of the contaminated sample was recovered entirely by washing with hydrofluoric acid and substantially by an ultrasonic treatment in soapy water. Scanning electron microscopy and electron microprobe analyses indicated thick, crystalline silica deposits on the catalyst surface. The deposits may have very high BET surface areas, so that the measurement of the catalyst area by itself would be misleading in determining the thermal deterioration of the catalyst. In addition to catalyst concerns, another major consequence of Si contamination is the deterioration of the exhaust gas oxygen (EGO) sensors.