We prepared a number of immiscible polymer blends, measured their microhardness and compared the results with theoretical predictions. The polymers used for the blends preparation were: low-density polyethylene (LDPE), high-density polyethylene (HDPE), isotactic polypropylene (PP), cycloolefin copolymer (COC), and EPDM terpolymer (EPDM). All blends reported in this work (PP/COC, HDPE/COC, and LDPE/PP) were prepared at various compositions, using melt-mixing followed by injection molding. The PP/COC and PE/COC blends exhibited processing-induced fibrous morphology of COC fibers. The HDPE/COC blends were made in three batches with different viscosities of HDPE. The LDPE/PP blends were both non-compatibilized and compatibilized with EPDM. Characterization of the blends included electron microscopy (SEM and TEM), tensile tests (according to ISO 527-2) and microhardness experiments (Vickers method; load 50 gf applied for 6s). The microhardness (MH) of a polymeric material is a complex property related to its composition and macromechanical properties. As for the macromechanical properties, the fundamental formula is Tabor's relation (MH = 3 x SY, where SY is the macroscopic yield stress). As for the composition, the classical predictions of MH are based on additivity law, i.e. the total microhardness of the system is proportional to the weight fractions of the individual components. In this contribution, we demonstrate that microhardness of polymer blends can be predicted with a simple equivalent box model (European Polymer Journal 48 (2012) 2031–2039) more accurately than with the widely-used additivity law. The work was supported through grants GACR P205/10/0348, TACR TE01020118, and RVO:61389013.