Measurements of density of aqueous solutions of decylammonium chloride (DACl) and cesium perfluorooctanoate (CsPFO) in the range of concentration from 0 to 50% by weight were carried out at 20, 30 and 40 o C. From these results the apparent molal volume of aqueous solutions of DACl and CsPFO have been determined as a function of concentration and temperature. On the basis of the apparent molal volume at concentration higher than the critical micelle concentration (CMC), the apparent molal volume at the CMC and the volume change upon micellization (ΔV) have been determined. The ΔV has also been evaluated from the calculation of the partial molal volumes above and below the CMC. There is a good agreement between the average values of ΔV for DACl and CsPFO at a given temperature determined in both methods. However, ΔV for CsPFO depends on concentration, particularly in the region of the highest concentration. It was stated that ΔV for DACl and CsPFO depends only slightly on the temperature and that ΔV for CsPFO is higher than for DACl. It was also found that the volume of DACl and CsPFO molecules at the CMC is only barely higher than their volumes in solid state, and at concentrations higher than the CMC their volume is close to that calculated in an hypothetical liquid state.