A curing or bridging composition is useful for effecting the crosslinking of phenolic resoles and the formation of novolaks. The composition comprises from 1% to 99% by weight of partially or totally sulfonated calixarenes and from 99% to 1% of partially or totally sulfonated linear phenolic oligomers, optionally together with partially or totally sulfonated phenol. To the contrary of conventional catalysts for the crosslinking of resoles and the formation of novolaks, the composition is chemically incorporated into the molecular structure of the final products; especially the sulfonated calixarenes confer new, surprisingly improved properties to phenolic resins and novolaks and sensibly accelerate the condensation reactions.