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Six different vegetable oil esters (coconut ethyl ester, used hydrogenated soy methyl ester, rapeseed ethyl ester, mustard ethyl ester, safflower ethyl ester, and a commercial methyl ester of soy oil) were selected to represent a range of iodine numbers from 7.88 to 133. These vegetable oil esters were tested neat and in 20% biodiesel/80% diesel blends in comparison with low sulfur diesel fuel for the effect on regulated emissions. The test vehicle was a pickup truck with a 5.9 L turbo-charged and inter-cooled direct injection diesel engine. The emissions tests were conducted at the Los Angeles County Metropolitan Transit Authority Emissions Testing Facility on a chassis dynamometer. It was found that lower iodine numbers correlated with reduced nitrogen oxides (NOx). As iodine number increased from 7.88 to 129.5 the NOx increased 29.3%. Fatty acids with two double bonds appeared to have more effect on increasing NOx emissions than did fatty acids with one double bond. Changes in carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM) were not linearly correlated with iodine number. It is apparent that the type of feedstock oil affects the characteristics of the biodiesel fuel. The most obvious difference is that the pour point changes with fatty acid composition, however, other fuel characteristics, some of which effect combustion, are also changed. This article reports on a study of biodiesel iodine number on changes in regulated emissions. The results of this and similar studies provide information for developing triglycerides specifically for optimum use in biodiesel. Modern chemical processes and/or plant breeding should make this possible.