Recently, pre-activation was determined to be an important reason for selectivity problems in the processing of a Boliden sulphide ore. The Petiknas South ore was investigated by TOF-LIMS (time of flight laser ionisation mass spectrometry) and it is now clear that the problems in the selectivity of copper-lead bulk flotation against pyrite for this ore are caused by pre-activation. Preliminary investigations into possible methods of countering pre-activation have been carried out.