Electrodes have been processed as composites from mixtures of single-walled carbon nanotubes (SWCNT) and a commercial activated carbon (MAXSORB). SWCNT shows higher electric conductivity and lower specific capacitance compared to MAXSORB. The relative content of the two carbons has been varied in the range 0-100 wt %. With the processed electrodes supercapacitor cells have been assembled. The cells have been studied at room temperature by ac and dc measurements. The dependences of the real capacitance (C') and the imaginary capacitance (C '') as functions of the angular frequency (omega) have been analyzed on the basis of a model we propose. Cell resistance and cell capacitance decrease as the SWCNT content increases according to the percolation model and rule of mixtures, respectively. Two relaxation times, one longer (of the order of 1-10 s) and the other shorter (of the order of 10(-6)-10(-5) s) have been found and discussed. The capacitance measured by dc has been compared with the capacitance measured by ac, and a discussion has been set up on the basis of the longer relaxation time determined by ac and the discharge time determined by dc. Power attains a maximum value for a certain SWCNT content. (c) 2007 The Electrochemical Society.