Blends of polypropylene (PP) and thermoplastic elastomers (TPE), namely SBS (styrene-butadiene-styrene) and SEBS (styrene-ethylene/1-butene-styrene) block copolymers, were prepared to evaluate the effectiveness of the TPE type as an impact modifier for PP and influence of the concentration of elastomer on the polymer properties. Polypropylene homopolymer (PP-H) and ethylene-propylene random copolymer (PP-R) were evaluated as the PP matrix. Results showed that TPEs had a nucleating effect that caused the PP crystallization temperature to increase, with SBS being more effective than SEBS. Microstructure characterization tests showed that in most cases PP/SEBS blends showed the smallest rubber droplets regardless of the matrix used. It was seen that SEBS is a more effective toughening agent for PP than SBS. At 0degreesC the Izod impact strength of the PP-H/SEBS 30% b/w blend was twofold higher than the SBS strength, with the PP-R/SEBS 30% b/w blend showing no break. A similar behavior on tensile properties and flexural modulus were observed in both PP/TPE blends. Yield stress and tensile strength decreased and elongation at break increased by expanding the dispersed elastomeric phase in the PP matrix. (C) 2004 Wiley Periodicals, Inc.