An effective method was proposed to determine the two organic-phase rate constants of the primary and secondary reactions which have differences in rates. The goal was achieved by employing the reaction of two mixed I-alcohols and dibromomethane in an alkaline solution of KOH/chlorobenzene two-phase medium under phase transfer catalysis. A new product of unsymmetric acetal was obtained in this work. The intermediate ROCH2Br (alpha-haloalkyl ether) was not detected during or after the reaction when using high reactive alcohols. This result indicated that ROCH2Br was more reactive than the organic reactant CH2Br2. This high reactive alpha-haloalkyl ether implied that the rate constants of secondary reactions are larger than those of the primary reactions. The resistance of mass transfer of the catalyst ((C4H9)(4)NBr, QBr) and the active catalyst ((C4H9)(4)NOR, QOR, R : C4H9 and C8H17) transferring from the aqueous phase to the organic-phase and vice versa were found to be negligible. The organic-phase reaction is a rate-determining step of the phase transfer catalytic reaction. A measured constant concentration of tetrabutylammonium alkoxide (QOR) during the reaction leads to the application of pseudo-first order rate law. The reaction rate constant of ROCH2Br acts 10(4) larger than that of CH2Br2.