Solid acids are used as catalysts for the polymerization of a feed stream containing at least one of pure monomer, C5 monomers, and C9 monomers to produce hydrocarbon resins. Freely-associated water may be removed from the solid acid catalyst prior to use. Resins with softening points (Ring and Ball) in the range of about 5 DEG C to 170 DEG C can be prepared. These catalysts offer advantages over the traditional Friedel-Crafts polymerisation catalysts since the acid sites are an integral part of the solid. The solid acid catalysts are relatively nonhazardous, reusable catalysts which eliminate or at least reduce contamination of the resulting resin products with acid residues or by-products.