Acid modified C.sub.5 hydrocarbon resins are disclosed. The resins are predominantly piperylene resins with isobutylene. The resins are acid modified with from about 0.1 to 30 weight percent maleic anhydride to provide an acid-modified resin with a acid number from about 30 to about 170 mg KOH/g resin. The acid-modified resins have a mettler drop softening point from about 40.degree. C. to about 140.degree. C. The acid modified resins are useful as a natural rosin alternative.