An in-situ radical-quenching experiment was performed in an e.s.r. microwave cavity by using a solution flow system. Diphenylmethyl radicals produced from pyrolysis of 1,1,2,2-tetraphenylethane were quenched by several H-donor solvents. Among the solvents used, indan was the fastest in quenching, followed by hydrophenanthrene, tetralin, mesitylene and cumene in decreasing order of quenching rate.