The influence of FA ester chemical structures on the rheology and crystallization temperature of those compounds was evaluated using methyl, n-butyl, n-octyl, and 2-ethyl-1-hexyl FA esters with different chain lengths and different degrees of unsaturation. The rheological properties were analyzed in a highprecision rheometer at various temperatures, and the crystallization temperatures were determined by DSC. Esters produced from the esterification of pure FA and from the transesterification of vegetable oils (i.e., soybean, corn, linseed, and babassu coconut oils) were evaluated. The length of the FA chain was shown to have a marked influence on the viscosity and crystallization temperature of the systems, whereas branching affected only the crystallization temperature to a significant extent. The viscosity and crystallization temperature of the systems were also influenced by the degree of unsaturation. One double bond was shown to increase viscosity, whereas two or three double bonds caused a decrease in the viscosity of the systems. Unsaturation lowered the crystallization temperature in all cases, regardless of the number of double bonds. From all the oils studied, methyl esters from babassu coconut oil presented the lowest crystallization temperatures.Vegetable oil esters have been widely used in the chemical industry. Their current use for biodiesel-a useful renewable energy source-is a likely substitute for diesel oil in future years because of the imminent exhaustion of petroleum reserves worldwide (1). Many studies have aimed to obtain biodiesel with properties (e.g., flash point, cetane number, viscosity, cloud point, pour point, and crystallization temperature) suitable for utilization in diesel engines (2-5).The viscosity of a fluid plays a major role in its pumping and flow within an engine. According to Ma and Hanna (6), the desirable viscosity for diesel oil at 37.8°C ranges from 1.9 to 4.1 cSt. At this temperature, soybean oil exceeds the optimal range, giving values as high as 32.6 cSt. For biodiesel, transesterification is an efficient method of reducing the high viscosity of vegetable oils while also improving other desirable properties (6). Methanol is one of the preferred alcohols used to transesterify vegetable oils and produce biodiesel. Generally, methyl esters have a Newtonian behavior within typical working temperatures. However, preliminary results from our laboratory have shown that methyl esters from soybean oil present a pseudoplastic behavior when temperatures reach values below 5°C, presenting high viscosity under low shear rates and low viscosity under high shear rates. Similar results were also obtained by Srivastava and Prasad (5) with methyl esters from soybean and mustard seed oils, confirming our data. This high viscosity at lower temperatures could be a result of micro-crystal formation and would cause serious problems in fuel lines and in engine filters.Therefore, the crystallization temperature is an important factor to be considered. DSC is an efficient method for invest...