Biodiesel obtained by transesterification of soy oil with methanol was isomerized in the liquid phase at 125-275 degrees C using solid acid catalysts. The purpose of this work was to improve the cold flow properties of biodiesel by inhibiting the formation of crystals. Crystallization was analyzed by means of the measurement of the cloud point of the fuel. Tested catalysts were SO42- - ZrO2 and H-mordenite. At a temperature of reaction (T-r) lower than 125 degrees C, the catalytic activity was negligible. At T-r > 200 degrees C, the reaction rate was greatly accelerated, but the cetane number was also decreased and the amount of coke deposits was greatly increased. While reacting biodiesel at 150-200 degrees C, a small decrease (4-6.5 degrees C) of the cloud point was obtained without a meaningful decrease of the cetane index. Best results were obtained with SO42- - ZrO2 at 125 degrees C. Isomerization had both positive and negative effects on fuel properties: it reduced the cloud point but also increased the viscosity and decreased the cetane index. Due to the different reactivity between the saturated fraction (estearates, palmitates) and the unsaturated one ( linoleates, linolenoates, oleates), an adequate control of the degree of isomerization was only possible by performing a previous fractionation step. A good combination was obtained by reacting the saturates rich fraction at 200 degrees C over H-mordenite and the unsaturated rich fraction at 150 degrees C over SO42- - ZrO2. The fuel obtained by mixing both reacted fractions had a cetane index of 49.5 and a cloud point of - 2 degrees C. However, this combined approach yields only a little benefit compared to those obtained with the single reaction over SO42- - ZrO2 at 125 degrees C.