Am in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) technique has been developed to study the adsorption of sodium dodecyl sulfate (SDS) onto pure, colloidal hematite as a function of both concentration and pH. The absorption bands of methyl and methylene groups are most reliable for quantifying adsorption unambiguously. The infrared spectral results have been correlated with zeta-potential studies to establish an absence of adsorption above the iso-electric point of 7.3, and to indicate that the mechanism of adsorption is physical, involving reversible electrostatic and hydrophobic forces. In the same way, there is no evidence for the existence of chemisorption. This is supported in the spectral results by the absence of shifts in relative band intensities and the absence of new adsorption bands. At low pH and high SDS concentrations, the adsorption is sufficient to cause a reversal in sign of the zeta-potential. Spectral absorption measurements should be made under in situ conditions to obtain unambiguous results. Studies of adsorption on natural hematite indicate a high depletion of SDS from solution at low pH; the measurements are consistent with precipitation losses due to the presence of calcium and ferric ions. This precipitation leads to a drop in flotation recovery. At high pH, adsorption occurs onto mineral particles which have become coated with carbonate mineral, leading to a flat variation of recovery with pH. The results have enabled optimum flotation conditions to be defined. Recovery can be directly related to adsorption density at moderate pH below the iso-electric point. The optimum pH for flotation is 5; extremes of pH are detrimental to flotation recovery of pure mineral and should be avoided. Low pH causes surfactant precipitation, while high pH will lead to surface transformation of the mineral. A collector concentration of 0.05 times the critical micelle concentration, corresponding to partial monolayer coverage on the hematite surface, is sufficient to ensure good recovery.