When the impeller Reynolds number is sufficiently high, so that the turbulence in a stirred tank is fully developed, the macroscopic flow characteristics, e.g. velocity distribution, power consumption and bulk blending time, are independent of viscosity. However, the fine-scale characteristics, near the Kolmogorov microscale, are influenced by viscosity and an effect on micromixing must be expected. This was confirmed as the product distribution of a pair of parallel reactions in a semi-batch reactor varied with the feed time, stirrer speed, feed location and viscosity. It was also consistent with the engulfment model of micromixing.