Influence of C g -OH on pyrolytic cleavage mechanism of b-ether-linkage in lignin dimer was studied in N 2 with C g -deoxy-type dimers, which have various p-substituents (-H, -Cl, -OCH 3 ) at their C b -phenoxy groups. Reactivities at 400 8C and the influence of radical scavenger (tetralin) indicated that C g -deoxy types, especially in phenolic forms, are degraded through radical chain mechanism. Furthermore, substituent effects on their reactivities under excess amount of tetralin as discussed with Hammett's substituent constant (s p ) and bond dissociation energy (BDE)reducing parameter (DBDE) revealed that this chain reaction is initiated with the radical species formed through C b -O bond homolysis. These chain reactions were only effective in a closed-type reactor and not effective in an open-type reactor. Contrary to this, C g -OH type in phenolic form was degraded at 250 8C, which is much lower than that (400 8C) of the C g -deoxy types, and the reactivity was not influenced by tetralin. These results indicate that introducing OH at C g changes the b-ether cleavage mechanism in phenolic form from radical chain to quinone methide mechanism.