In an experimental study of polyethylene (PE) and poly(methyl methacrylate)(PMMA) blends, complex viscosityη*(ω)=η′−iη″and non‐Newtonian viscosityη(γ̇)were obtained with a Weissenberg rheogoniometer operating in a cone‐and‐plate mode at 160 °C. Blends were prepared by a standardized melt mixing for volume fractions φ fromφ=0(PE)to 1.0 (PMMA) at increments ofΔφ=0.1.The PMMA was more viscous than PE at all conditions of ω andγ̇,their viscosity ratio being 5.9 in the Newtonianη0limit. Scanning electron micrographs were taken of freeze‐fracture planes in all samples prior to testing. A maximum in both η andη′was found nearφ=0.9and an apparent step increase of viscosity level in the vicinity of 0.5–0.6 at all ω andγ̇.The maximum is quantitatively explained in the Newtonian limit by well‐established viscosity predictions for dilute two‐phase emulsions with spherical droplets; this model is equally successful forφ=0.1(the other dilute regime) and, in both cases, valid also within 5–10% for a substantial range of ω‐ andγ̇‐dependent data. The step increase is attributed to phase inversion, but smooth interpolation betweenφ=0.1and 0.9 is found to yield rather good empirical predictions for intermediate compositions without recourse to microstructural information. Rather conventional behavior is seen in comparisons ofη(γ̇)withη′(ω)and|η*|,the latter upholding the Cox‐Merz approximationη=|η*|rather well. Exceptions to these are noted in the intermediate φ range, but only in theη0limit after high‐γ̇testing, and the discrepancies are suspected to arise from microstructural alterations during steady shear.