A prepolymer comprising an acid group is made by a process comprising the step of contacting: (i) a di-isocyanate, (ii) a polyol containing an acid group, and (iii) a polyol without an acid group, the contacting conducted under reaction conditions and in a solvent consisting essentially of: (A) a first component consisting essentially of at least one glycol ether ketone, and (B) optionally, a second component consisting of at least one aprotic glycol ether. The prepolymer is useful in the preparation of water-based polyurethane dispersions.