In this work, the thermal stability of the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium ethylsulfate (EMIMEtSO4) is investigated using infrared (IR) spectroscopy. Quantitative IR absorption spectral data are measured for heated EMIMEtSO4. Spectra have been collected between 25 ℃ and 100 ℃ using a heated optical cell. Multiple samples and cell pathlengths are used to determine quantitative values for the molar absorptivity of EMIMEtSO4. These results are compared to previous computational models of the ion pair. These quantitative spectra are used to measure the rate of thermal decomposition of EMIMEtSO4 at elevated temperatures. The spectroscopic measurements of the rate of decomposition show that thermogravimetric methods overestimate the thermal stability of EMIMEtSO4.