Infrared spectroscopy is one of the most fundamental tools of analysis available, and it has the ability to provide most, if not all, of the data required from a used lubricant. In the past successful, yet superficial, application of the technique has been made for condition monitoring of oils during service. The full potential of the application, however, has been limited by the inability to access the total information content of the infrared spectrum. Modern computer-assisted instrumentation now provides the sensitivity and the means to extract the data. This paper will describe the technique and will illustrate how it can be applied to the analysis of used lubricants.