Iodine number is used as a measure of the fuel unsaturation of vegetable oils and fatty acid methyl esters. However, its relevance as a measure of total unsaturation of petroleum fuels like diesel, Jet A and their blends with biodiesels is debatable due to the significant differences in the reactivity of iodine with petroleum fuels. Bromine number, used as a measure of aliphatic unsaturation in petrofuel samples, does not account for the aromatic unsaturation from petroleum fuels. Hence, a common parameter that is relevant for both biodiesels and petroleum fuels needs to be identified to quantify the fuel unsaturation. A parameter, termed "Degree of Unsaturation (DOU)," which accounts for the total unsaturation of the fuel from all sources such as double and triple bonds, aromatics and other ring structures irrespective of the families of the fuels (alkanes, alkenes, alkynes, aromatics, ether or ester) that has been used in organic chemistry literature is proposed in this work and identified as a potential indicator of NOx emissions from biodiesel blends. DOU is observed to strongly correlate with cetane and iodine numbers. Furthermore, relationships between DOU and the NO emission index on a mass basis (EINO) in prevaporized laminar flames of neat fuels such as soy methyl ester (SME), canola methyl ester (CME), rapeseed methyl ester (RME), palm methyl ester (PME), diesel, Jet A and petroleum/biodiesel blends at various equivalence ratios are presented. The NO emission index of flames of biodiesel/petroleum blends was found to increase with DOU, but with varying trends depending on their families of origin. The effects of DOU on Elm were significantly influenced by the equivalence ratio, with the maximum influence at an equivalence ratio of 1.2. At the equivalence ratio of 1.2, EINO increased from 2.4 g/kg at a DOU value of 1.7-4.4 g/kg at a DOU of 3.0 DOU provides a common platform to compare and quantify the effects of fuel unsaturation across different fuel families and can be employed as an indicator of NOx emissions. DOU can be evaluated based on the average molecular formula of the fuel alone without involving complex and expensive experimental procedures such as those involved in the measurement of iodine number. (C) 2016 Elsevier Ltd. All rights reserved.