A dispersing agent for use in a thermosetting composition, a thermosetting composition containing said dispersing agent, and application thereof to thermosetting composition having no yield point. The dispersing agent acting on mineral fillers and used in filled acrylic or polyester resin-based thermosetting compositions belongs to the family of organophosphates corresponding to the general formula: ##STR1## where: A is a branched or unbranched polyaryl group,x and y are whole numbers between 0 and 100 such that x+y is a whole number greater than 40 but not greater than 100, ##STR2## B and E are independently a branched or unbranched polyaryl group, or an alkyl, aryl, arylalkyl, alkylaryl, alkanoyl, or amine radical, and in B and E, and R.sub.1 and R.sub.2 may be identical or different, x1, y1, x2 and y2 are whole numbers, and the sums x1+y1 and x2+y2 are not greater than 100.