Abstract: A polyalkylene glycol ether phosphate ester, or salt thereof, having the formula:R.sub.w --P(O)(OH).sub.3-w (I)wherein w is an integer from 1 to 3, andR is an oil-soluble polyether moiety having the formula:R.sup.1 X[(C.sub.x H.sub.2x O).sub.n (C.sub.y H.sub.2y O).sub.p ].sub.m(II)wherein R.sup.1 is either an alkyl or an alkyl phenyl group having from 7 to 30 carbon atoms,X is either O, S, or Nx is an integer from 2 to 4,y is an integer from 6 to 30,m is 1 when X is O or S and m is 2 when X is N,n and p are such that the polyether moiety contains between 1 and 35% by weight of (C.sub.y H.sub.2y O) units and between 35 and 80% by weight of (C.sub.x H.sub.2x O) units, and the molecular weight of the polyalkylene glycol moiety is between 500 and 5000 is disclosed,The phosphate ester can be prepared by reacting a polyether with a phosphating agent, e.g. phosphorus pentoxide and water. Lubricating oil compositions comprising the phosphate ester or salt thereof are also disclosed.