High-density polyethylene (HDPE) was blended with "reinforcing" cycloolefin copolymer (COC) in order to produce polyolefin materials with increased stiffness, yield and tensile strength. Experimental data on tensile modulus E-b, creep modulus E-bcr, storage modulus E-b' loss modulus E-b", yield strength S-yb, and tensile strength S-ub of blends are in plausible accord with their simultaneous prediction based on a predictive format that operates with a two-parameter equivalent box model and the data on the phase continuity of components obtained from modified equations of the percolation theory. Dependencies of these mechanical properties on blend composition indicate the critical volume fraction v(2cr) = 0.16 of COC. Interfacial adhesion in the HDPE/COC blends is strong enough to transmit acting stress up to the break point. Strain at break, tensile energy to break and tensile impact strength show conspicuous drops in the interval 15-25% of COC in the blends, during which COC starts to form a co-continuous brittle component. Further growth of COC fraction accounts for reduction of blend ultimate properties to values typical of brittle polymers. However, tensile impact strength shows a local maximum at HDPE/COC = 25/75, which probably corresponds to COC toughened with HDPE particles. (c) 2005 Society of Plastics Engineers.