Potentiometric titrations, solubility measurements, ultraviolet (UV) and Fourier Transform Infrared (FTIR) spectroscopy were used to investigate the solution properties of the copper sulfide collectors, O-isopropyl-N-ethyl thionocarbamate (IPETC) and O-isobutyl-N-ethoxycarbonylthion-ocarbamate (IBECTC). The molar absorptivity spectra of IBECTC and IPETC were monitored as a function of pH. IPETC exists as a single, spectroscopically distinct, neutral species in aqueous solution to a pH value of at least 10.5, Three clearly defined isosbestic points appear in the molar absorptivity spectra of IBECTC with increasing solution pH and indicate the presence of at least two spectroscopically distinct species, representing acidic and basic forms of the collector. The pK(a) (8.1) and molar absorptivity spectra of the basic species of IBECTC were calculated from the UV spectra. A similar pK, value for IBECTC was found by potentiometric titration. Solubility studies confirmed these observations, demonstrating that the basic form of IBECTC is more than ten times more soluble than the acid form. Based on FTIR studies, a molecular configuration of IBECTC(-) is proposed. Microflotation studies of chalcopyrite, performed as a function of pH in the presence of these collectors, showed a recovery-pH response which correlated with the collectors＇ solution behaviour.