Abstract: An adhesion promoter, suitable for improving the adhesion of composite dental material to tooth enamel, comprises a solution in a volatile organic solvent therefor, such as a lower alkanol, of a polyethylenically unsaturated monophosphate or salt thereof, containing a monophosphate radical and at least three ethylenically unsaturated groups (preferably terminal ethylenically unsaturated groups) per molecule. Preferred polyunsaturated phosphates are monophosphates of polyhydric alcohols containing at least four hydroxyl groups per molecule in which at least three hydroxyl groups are esterified with an ethylenically unsaturated carboxylic acid, and especially preferred polyunsaturated phosphates (which are provided as new compounds) are those of the formula: ##STR1## in which: R is an aliphatic (optionally interrupted by one or more oxygen atoms) cycloaliphatic or aryl radical having a valency of n+1 and containing from 4 to 16 carbon atoms,R.sup.1 is a hydrogen atom, alkyl C.sub.1 -C.sub.