PURPOSE: To obtain an organosiloxane fuel additive improved in the yield and quality of the product by mixing a specified siloxane component with a specified chlorosilane component and FeCl3 catalyst and feeding gaseous hydrogen chloride into the obtained mixture.

CONSTITUTION: An intended compd. of formula II is obtd. by mixing a siloxane component (R3Si)2O with a chlorosilane component of formula I and FeCl3 catalyst and then feeding a catalytic amt. (0.05 to 1 wt.%) of gaseous hydrogen chloride to initiate the reaction and carrying out the reaction until the mixture has been completedly reacted under stirring and, if necessary, under temp. control by cooling or heating. This compd. is used as a fuel additive for fuel oils and/or heavy oils. In the formulae, R is aryl optionaly substd. by 1-18C alkyl or the like optionally substd. by an ester group or the like; R" is H and/or R optionally substd. by a group of formula III (wherein m is 0, 1 or 2), or the like; and n is 2 to 4.