Upon addition of small quantities of a second phase the mixture shows a droplet/matrix structure, changing into a co‐continuous one at the percolation threshold, ϕc≂0.16. The level of phase co‐continuity reaches a maximum at the phase inversion concentration, ϕ2I, where the viscosities of a system in which polymer‐1 is dispersed in polymer‐2 and the other where polymer‐2 is dispersed in polymer‐1 are equal. Thus ϕ1I can be expressed as function of viscosity ratio, λ=η1/η2, with intrinsic viscosity and maximum packing volume fraction as parameters. Next, it was postulated that in immiscible polymer blends the viscosity‐concentration dependence is determined by two mechanisms: an interlayer slip and the emulsionlike viscosity increase culminating at the inversion concentration, ϕ2I. The derived two‐parameter equation was found to describe the observed η versus ϕ dependence quite well. The parameters, calculated from the least‐squares regression fit of experimental data to the two‐parameter equation, were found to provide insight into the flow mechanism.