Petroleum resins having a softening point from 10.degree. C. to 80.degree. C. obtained from C.sub.5 olefins and diolefines and one or more monovinyl aromatic compounds which contain from 10 to 30 wt. % of the aromatic are useful as tackifiers for carboxylated styrene butadiene copolymer to produce pressure sensitive additives especially aqueous based adhesive when the resin is in the form of an emulsion which is also claimed.