Novel addition curable phenolic resins bearing terminal ethynyl groups anchored to benzene ring through a phenyl azo linkage were realized by the coupling reaction between novolac and 3-ethynyl phenyl diazonium salt. The diazo-coupling occurred to a maximum of 50 mol%. The apparent molecular weight, determined from gel permeation chromatography showed a downward drift with increase in degree of acetylene substitution. The resin underwent curing in a broad temperature range 140-240 degreesC. Analyses using a model compound indicate that the curing occurs via various addition polymerization of acetylene groups. The thermal stability and anaerobic char yield of the polymers increased proportional to the crosslinking and were considerably superior to those of a conventional resole. Isothermal pyrolysis studies implied the possibility for the decomposition occurring mainly by loss of nitrogen gas, and hydrocarbon to form mostly an amorphous char.