Solid/liquid- and vapor-phase interactions between cellulose- and lignin (Japanese cedar milled wood lignin)-derived pyrolysis products were studied under the conditions of N2/600 °C/40–80 s. A dual-space closed ampoule reactor was used to eliminate the solid/liquid-phase interactions, and careful comparison of the resulting data with those of the pyrolysis of the mixed samples gave some insights into the solid/liquid- and vapor-phase interactions separately. With the solid/liquid-phase interactions, the tar yields from both cellulose and lignin increased with the decreasing yields of the char fractions in a short pyrolysis time of 40 s (primary pyrolysis stage). Most of the identified tar components from cellulose and lignin increased in their yields. The vapor-phase interactions were significant at a longer pyrolysis time of 80 s (secondary reaction stage) when the methoxyl groups of the lignin-derived volatiles were cleaved homolytically. The vapor-phase interactions accelerated the gas formation from the cellulose-derived volatiles with suppressing the vapor-phase char formation of the lignin-derived volatiles. The yields of methane and catechols from lignin also increased greatly instead of the formation of o-cresols. Most of these influences are explained with a proposed interaction mechanism, in which the cellulose-derived volatiles act as H-donors while the lignin-derived volatiles (radicals) act as H-acceptors.