Infrared spectroscopy is one of the most important analytical tools for the study of lubricant degradation and for condition monitoring of oils during service. The application of computer post processing to an infrared spectrum provides a means for the extraction of oil degradation data from a used diesel oil sample without need for the separation of soot and other dispersed insoluble material. Data acquired by this type of approach are representative of the actual condition of the lubricant and they provide a basis for monitoring oil degradation insitu. This paper will discuss physical and chemical characteristics pertinent to the performance and condition of used diesel oils that are reflected in the infrared spectrum. It will also provide a summary of the results from a statistical study of infrared data accumulated from oils taken from five different types of diesel engine in current service with military equipment.