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Kinematic viscosities were measured for ethanol and n-butanol blends with diesel and biodiesel fuels, showing that increasing alcohol contents contributed to a non-linear decrease in the viscosity. Only ethanol blends with alcohol content lower than 36% (volume basis) fulfill diesel fuel quality standard, whereas the whole range of n-butanol blends fulfills this standard. Generalized correlations were tested for the estimation of the viscosity of ethanol and butanol blends with diesel and biodiesel fuels in the whole range of alcohol contents. Interaction coefficients were required for an adequate estimation due to strong molecular interactions. Two-parameter equations did not improve, and three-parameter equations improved only slightly, the estimations with respect to the Grunberg-Nissan equation, with only one interaction coefficient. This interaction coefficient decreases when the number of carbon atoms in the n-alcohol molecule increases for both diesel blends and biodiesel blends because the non-polar part of the molecule becomes dominant against the polar hydroxyl group. Original correlations were proposed for the modeling of the interaction coefficient in alcohol-diesel and alcohol biodiesel fuels as a function of the carbon number of the alcohol. Other n-alcohols (methanol, propanol and n-pentanol) were also tested to extend the validity of the correlation proposed. These correlations are useful for the design of transport equipment or pipelines in a scenario where alcohols may constitute a fuel component in diesel blends. Also the decision on feasible blends that fulfill the narrow viscosity ranges set by fuel standards (to avoid loss of vehicle operability) will benefit from the proposed correlations.