Technical Abstract: Biodiesel made from different source materials usually have different physical and chemical properties and the concentration of biodiesel in biodiesel-diesel blends varies from pump to pump and from user to user; all these factors have significant effects on performance and efficiency of engines fueled with biodiesel. To address these challenges, models based on near-infrared spectroscopy were developed for relatively inexpensive and rapid on-line measurement of the concentration and specific gravity of biodiesel-diesel blends. Methyl esters of five different oils—soybean oil, canola oil, palm oil, waste cooking oil, and coconut oil—and two different brands of commercial-grade No. 2 on-highway diesel and one brand of off-road No. 2 diesel were used in the calibration and validation processes. The predicted concentration and specific gravity of the biodiesel-diesel blends were compared with the actual values. The maximum and average prediction errors of biodiesel concentration were 5.2% and 2.9%, respectively, from the biodiesel type-specific model. For the general model, the maximum and average prediction errors were 7.5% and 2.6%, respectively. The specific gravity prediction model had a maximum prediction error of 0.005 and an average error of 0.002.