This paper describes laboratory and plant based investigations on methods to increase fine copper recovery during flotation treatment of Ok Tedi porphyry sulphide ores. Currently the Ok Tedi concentrator treats a mixed porphyry/skarn ore blend, which is ground to a d(80) of 180 mu m with flotation carried out using a dithiophosphate/monothiophosphate collector mixture, at pH 11.5. Plant survey data showed that recovery of fine copper minerals in the Ok Tedi concentrator was at moderate levels, with only 81% of the -10 mu m copper in the feed recovered in roughing and scavenging stages. Pulp and surface chemical measurements during the survey period suggested that oxidation of copper minerals occurred during processing. Attention then focused on characterising the size-by-size laboratory flotation response of two porphyry sulphide ore samples, as an initial step towards developing methods to increase fine copper recovery. These tests showed that for an ore sample in which the predominant copper mineral was digenite, -10 mu m copper recovery was only 40%. By contrast, for an ore sample in which the predominant copper mineral was chalcopyrite, -10 mu m copper recovery was 80%. Low recovery of fine (-10 mu m) copper in the digenite ore sample was linked to the degree of oxidation of this mineral during laboratory grinding and conditioning. Controlled potential sulphidisation experiments were undertaken to counter the adverse effects of digenite oxidation. The electrochemical potential of sulphidisation was optimised. These experiments showed clear increases in fine copper recovery for the digenite ore sample and, to a lesser extent, the chalcopyrite ore sample. Results from plant trials of sulphidisation on a variety of porphyry sulphide ore blends are discussed. The size-by-size copper recovery in the Ok Tedi concentrator with and without sulphidisation treatment during two plant surveys are discussed.