Abstract: A highly reactive aromatic hydrocarbon-formaldehyde resin having a diarlymethane content of not more than 5% by weight, essentially containing reactive groups of any one of CH.sub.2 OH, CH.sub.2 OCH.sub.3 and CH.sub.2 OCH.sub.2 OCH.sub.3 at both molecular terminals and having a xylenol value of at least 15 mol/kg as an index of reactivity with a compound having active hydrogen atoms is prepared by reacting an aromatic hydrocarbon such as xylene, mesitylene or naphthalene with formaldehyde at a molar ratio of the formaldehyde to the aromatic hydrocarbon of 2.0 to 5.0, a sulfuric acid concentration of 15 to 35% by weight in an aqueous layer and a reaction temperature of 80.degree. to 110.degree. C. while controlling a formaldehyde conversion within a range of 50 to 70% by mole over the range 4 to 8 hours.

Abstract: A process for the preparation of a modified aromatic hydrocarbon resin which comprises reacting an aromatic hydrocarbon-formaldehyde resin with an unsaturated polybasic acid or an anhydride thereof, together with a saturated polybasic acid or an anhydride thereof if necessary, in the presence or absence of water, while introducing steam into the reaction system to remove by-produced formaldehyde out of the reaction system, and reacting the formed reaction product with at least one dihydroxy compound selected from the group consisting of (A) a glycol and (B) a hydroxy-terminated oligomer obtained by reacting a glycol with an unsaturated polybasic acid or an anhydride thereof; the cured product of the modified aromatic hydrocarbon resin being excellent in mechanical strength and resistance to water and alkalis.

Abstract: A modified aromatic hydrocarbon resin is prepared by reacting an aromatic hydrocarbon-formaldehyde resin with an unsaturated polybasic acid or an anhydride thereof, if desired, together with a saturated polybasic acid or an anhydride thereof, while making water or water and an aliphatic alcohol present in the reaction system and removing the formaldehyde formed by the reaction. The cured product has a good mechanical strength and a high resistance to water and alkalis.