The polymerisation of styrene in the presence of methyl vinyl ketone (MVK) and triethylborane (BEt3) was studied. The mixture of these two compounds was found to form a bicomponent complex that functions as a transfer agent for growing polystyryl radicals (C-T = 0.7; benzene 60 degrees C). The analysis of the polymer chains formed by MALDI-TOF-MS showed that they were fitted with CH2-CH3 and CH2-CH2-C(O)-CH3 as end groups which obviously emanated from MVK and BEt3. In contrast, no such transfer occurred on using each of them as single compound.