Abstract: The tritiated alcohol quench method was used for determining the tritium number average molecular weight [M_n]_γ of polybutadiene prepared wtih lanthanide catalysts.The change in [M_n]_γ Wtih conversion for different lanthanide catalysts was studied. It was found that for certain lanthanide catalytic systems the values of [M_n]_γ of polybuta-diene were approximately equal to those obtained by using osmometry technique. It was shown that the tritium end group analysis could be used for determining Mn of polybuta-diene prepared with lanthanide catalysts.