Integration of phase change materials (PCM's) in textile fibers is a means to achieve thermo-regulating properties. When the body temperature goes up, the PCM melts and absorbs the heat from the body in the form of latent heat (cooling effect). When the temperature drops, the PCM crystallizes and the stored heat is released again (warming effect). PCM's can be integrated into textile fibers by means of melt spun bi-component fibers with a sheath/core structure. In this way PCM is trapped inside the fibers. Bi-component fibers with PA6 and PET sheaths were in this way successfully produced. Fibers containing up to about 40% PCM were produced showing heat of fusions up to about 80 J/g and tenacities above 15 cN/tex. The loss of PCM during use and repeated washing cycles was shown to be low.