In miscible or partially-miscible blends of semicrystalline polymer/non-cocrystallizing low molecular weight component, a decrease of the value of yield stress in comparison to reference (pure) polymer is usually observed. On the example of model polypropylene/nonadecane systems, the mechanisms responsible for the decrease of the yield stress have been identified. It has been proved that during the deformation of polypropylene/nonadecane blends containing low amount of nonadecane (up to 5 wt %) the reduction of the yield stress is caused only by the swelling of interlamellar regions. In the case of the systems containing a moderate amount of nonadecane (7-10 wt %), the reduction of the yield stress is caused by the swelling of interlamellar regions and the reduction of the sample cross-section effectively participating in transferring of tensile stress. In blends containing nonadecane in the amount of 15-30 wt % the reduction of the yield stress is caused by the swelling of interlamellar regions and strong asymmetrization of nonadecane microdomains, resulting in localizing the deformation along interspherulitic regions and a drastic reduction of the content of polypropylene matrix, effectively participating in transferring of tensile stress between adjacent spherulites. (C) 2018 Wiley Periodicals, Inc.