Blends based on recycled high density polyethylene (R-HDPE) and recycled poly(ethylene terephthalate) (R-PET) were made through reactive extrusion. The effects of maleated polyethylene (PE-g-MA), triblock copolymer of styrene and ethylene/butylene (SEBS), and 4,4'-methylenedi(phenyl isocyanate) (MDI) on blend properties were studied. The 2% PE-g-MA improved the compatibility of R-HDPE and R-PET in all blends toughened by SEBS. For the R-HDPE/R-PET (70/30 w/w) blend toughened by SEBS, the dispersed PET domain size was significantly reduced with use of 2% PE-g-MA, and the impact strength of the resultant blend doubled. For blends with R-PET matrix, all strengths were improved by adding MDI through extending the PET molecular chains. The crystalline behaviors of R-HDPE and R-PET in one-phase rich systems influenced each other. The addition of PE-g-MA and SEBS consistently reduced the crystalline level (X,) of either the R-PET or the R-HDPE phase and lowered the crystallization peak temperature (T-c) of R-PET. Further addition of MDI did not influence R-HDPE crystallization behavior but lowered the X, of R-PET in R-PET rich blends. The thermal stability of R-HDPE/R-PET 70/30 and 50/50 (w/w) blends were improved by chain-extension when 0.5% MDI was added. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1710-1719, 2009