The document describes an ionic liquid thermometer that uses ionic liquids as the temperature-sensitive fluid. It has advantages over mercury and ethanol thermometers like a wider temperature range from -100 to 400 degrees Celsius, lower toxicity, and easier calibration and disposal. The ionic liquids are inexpensive, thermally stable, readily biodegradable, and have complete toxicological data available. The inventors are Dr. Ramana Reddy and Dr. Robin Rogers who have patents and experience in materials engineering, chemistry, and ionic liquids.