The surface properties of celestite (SrSO4) and calcite (CaCO3) in aqueous solutions of the anionic collector sodium dodecylsulfonate (NaDDSO3) have been investigated making use of zeta potential, adsorption, infrared spectroscopy and microflotation techniques. A range of pH (3 to 11) and sulfactant concentration (10(-6) to 10(-3) M) were tested. The experiments were performed at a constant temperature (25 degrees C) and using regulated suspensions. The zeta potential studies showed that the isoelectric point (IEP) of celestite and calcite took place at pH 3 and 8, respectively. In the presence of dodecylsulfonate ion the IEP of both minerals shifted to lower pH values. The adsorption isotherms obtained for both minerals showed marginal adsorption up to certain surfactant concentration and precipitation of the salt (strontium or calcium dodecylsulfonate) onto the mineral surface beyond such a concentration. The microflotation studies showed that celestite was activated even at pH values where its surface was negatively charged. Compared to celestite, the activation of calcite was lower. It is suggested that the dodecylsulfonate ion adsorbed on the surface of celestite and calcite by a chemisorption mechanism giving rise, under certain circumstances, to the formation of the corresponding salt at the mineral-water interface.