Synthesis of lyotropic mixtures of potassium laurate, l-decanol, and water is presented, and the phase diagram of this ternary system is investigated in the vicinity of the nematic region. Several phases are identified and the corresponding concentration and temperature-induced phase transitions are studied. To prepare ferronematic liquid-crystalline samples, positively charged magnetic nanoparticles are introduced into the lyotropic matrix. Modifications of the phase behavior of the system induced by the particle incorporation are described.