A carboxyl modified, metal-free petroleum resin comprising a resin obtained by thermal polymerization of a feed containing cyclopentadiene and/or methylcyclopentadiene and/or their dimers or codimers having been reacted electrophilically with a saturated or ethylenically unsaturated higher fatty acid, said modified resin having an acid number less than 20, containing from 1 wt. % to 5 wt. % of oxygen and having a softening point of from 120.degree. C. to 200.degree. C.These resins may be prepared by heating a resin obtained by thermal polymerization of a feed containing the aforesaid cyclopentadiene and/or methylcyclopentadiene and/or derivatives thereof with said carboxylic acid to a temperature in the range of 200.degree. C. to 270.degree. C. and subsequently removing free acid to reduce the acid number to below 20.These resins may be used for offset printing inks, and suitable printing ink compositions comprise (a) 5 to 30% by weight of such a resin (b) 5 to 30% by weight of a drying oil, (c) 20 to 40% by weight of a pigment, (d) 0 to 10% by weight of an assistant, and (e) a solvent being the balance, the weight percentages of components (a), (b), (c) and (d) being based on the total weight of the composition.