Mixing of high-viscosity liquids (e.g. glycerol–water solutions) is challenging and costly and often requires employing active mixing methods. Two-phase flow micromixers have attracted attention due to their low cost, simple structure, and high performance. In the present work, we investigate the mixing of similar fluids with viscosities equal to or higher than that of water in a two-phase (gas-liquid) slug-flow micromixer, as an economical passive design. Various cases are studied, in which the liquid samples to be mixed are either water or glycerol–water solution. The performance of the proposed slug-flow micromixer is compared with that of a single-phase micromixer with similar geometrical configuration. We demonstrate that mixing efficiencies higher than 90% are attainable for species with viscosities of about 54% higher than that of water (O(10⁻³) kg m⁻¹ s⁻¹); a result that is not attainable in the corresponding single-phase micromixer. Moreover, a mixing efficiency of more than 80% is achieved at the outlet of the micromixer for solutions with viscosities of 160% higher than that of water.