Rheological properties of residual fuel oil, water-residual fuel oil emulsions, and their blends with off-grade methyl esters of fatty acids (MEFA) of rapeseed oil are studied in the 20-80 A degrees C range. It is demonstrated that addition of 1-10% of MEFA reduces the viscosity of residual fuel oil of the trademark M100 by 12-73% and addition of 1-3% of MEFA reduces the viscosity of water-residual fuel oil emulsions by 15-65%. Use of MEFA bioadditives and rapeseed oil is shown to be effective for modification of water-containing residual fuel oils.