Phthalide-containing diphenol 3-phenyl-3-(21,51-dihydroxyphenyl)phthalide was synthesised for the first time by the interaction of o-benzoylbenzoic acid chloride with hydroquinone in a dioxane solution in the presence of catalyst ZnCl2. Interaction of a mixture of the synthesised diphenol and phenol in different weight ratios (from 5:95 to 50:50) with formaldehyde in the presence of catalytic quantities of ammonia produced phenol–formaldehyde cooligomers of the resol resin type and heat-resistant crosslinked polymers based on them.