A grease composition containing: (a) an esterified polymer derived from monomers comprising: (i) a vinyl aromatic monomer; (ii) an unsaturated dicarboxylic acid anhydride or derivatives thereof; (b) a thickening agent; and (c) an oil of lubricating viscosity, wherein the esterified polymer contains titratable acid groups with a total acid number of at least about 4. The invention further relates to the process to make the composition and its use in grease to increase water resistance.