The effect of various types and concentrations of peroxides on the rheological, mechanical and morphological properties of a 60/40 NR/PP TPVs were investigated. The main factors that influenced the properties of the TPVs are: the typical crosslinking temperature, the crosslink efficiency and the relative amounts of decomposition products of each peroxide, in combination with the degradation of PP in the presence of peroxide. DCP and DTBPIB have typical crosslinking temperatures close to the mixing temperature, provide high crosslink efficiency, and highly reactive radicals. They provide high amounts of crosslinks in the rubber molecules. Moreover, in relation to decreasing viscosity of the PP via the PP degradation mechanism, the TPVs with DCP or DTBPIB give superior overall properties relative to DTBPH or DTBPHY as curatives.