The behaviour of fine particles in the flotation separation of minerals has become of rapidly increasing importance as new, fine grained deposits are exploited. Fine particles float poorly and less-selectively under normal flotation conditions. The behaviour of fine (<20 mu m) and coarse (38-75 mu m) sphalerite has been correlated between microflotation and aggregation studies. This study utilises two novel techniques for obtaining direct evidence of particle interactions within a conditioning pulp. On-line particle size distribution enables the onset of aggregation of fine particles under potation related conditions, during addition of reagents, to be determined Concurrent evidence of particle interactions has been observed using optical microscopy. Significant aggregation to 20-30 mu m aggregates during agitation conditioning before reagents was observed. Fine particles exhibit poor flotation response and some aggregation after Cu(II) activation alone. On addition of collector at tow pH, potation recovery and extent of aggregation (>100 mu m) increased dramatically. In the presence of coarse particles at low pH, a high percentage of fine particles were recovered indicating a fine-coarse particle aggregation (＇＇piggy-backing＇＇) mechanism occurring during conditioning with Cu(II) alone.