More than 25 PP/PS/SEP blends, where PP is isotactic polypropylene, PS is atactic polystyrene, and SEP is poly(styrene-block-ethylene-co-propylene), were prepared. The main objective of this study was to investigate the influence of PP/PS viscosity ratio, gimel((TM)), on the blends' morphology. It was shown that gimel((TM)) strongly influenced not only the overall morphology of the blends, but also the morphology of SEP, which exhibited as many as five different types of structure when blended with PP and/or PS. SEP was found an efficient compatibilizer of PP/PS blends as it decreased the average particle size in all studied systems. An interesting "by-product" of this work was the discovery of a brand-new type of polymer morphology, which was called morel structure. The characteristic feature of the morel structure was PS matrix compartmentalized by SEP. (c) 2006 Wiley Periodicals, Inc.