The present invention provides a poly(hydroxycarboxylic acid) amide salt derivative having formula (III): [Y—CO[O-A-CO]n—Z—R+]mpXq− (III) wherein Y is hydrogen or an optionally substituted hydrocarbyl group, A is a divalent optionally substituted hydrocarbyl group, n is from 1 to 100, preferably from 1 to 10, m is from 1 to 4, q is from 1 to 4 and p is an integer such that pq=m, Z is an optionally substituted divalent bridging group which is attached to the carbonyl group through a nitrogen atom, R+ is an ammonium group and Xq− is a non-sulphur-containing anion. Furthermore, the present invention provides a lubricating composition and a fuel composition containing the poly(hydroxycarboxylic acid) amide salt derivative and the use of the poly(hydroxycarboxylic acid) amide salt derivative for decreasing the phosphorus volatility of a fluid.