Soot was recovered from end-of-test (EOT) oils from several different Diesel engines. After resuspending the recovered soots in fresh high dispersancy engine oil, the infrared absorptivities of the soots were determined over a range of frequencies. On the basis of a statistical analysis of the data, recommended 1870 cm-1 absorptivities for soots from various engine tests are: GM 6.2 L, Mack T-8, Cummins L-10 HST, 59.0±0.5; GM 6.2 L, Mack T-8, Caterpillar 3176, 0.02% sulfur fuel, 58.2±0.4; Caterpillar 3176, 0.02% sulfur fuel and 0.2% sulfur fuel, 56.7±0.3; Caterpillar 3116, 53.3±0.4 and OM 602A, 47.8±0.2. In all cases, the units for the absorptivity are cm2/centigram. Over the range of 3800 cm-1 to 1870 cm-1, the dependence of absorptivity on frequency was linear for all the soots. Use of these absorptivities to determine soot concentrations in used engine oils is discussed. Of particular concern are the procedures required to obtain reproducible results. Special care must be taken to assure compositional uniformity of samples.