AbstractThe competitive, consecutive chemical reactions between 1‐naphthol and diazotized sulfanilic acid were utilized to study the mixing of a pulp fibre suspension in a 22 L stirred tank reactor. Mixing quality was determined from the distribution between the mono and bis substituted reaction products once a correlation was made for the adsorption of the product dyes onto the suspended fibres. The technique was found to be adequate for assessing micromixing and turbulence intensity within a fibre suspension provided the measured product distribution, Xs, was between 0.4 and 0.01. Thus the mixing conditions that could be assessed depended on both the energy dissipation within the mixer and the amount of the fibre present. For the experimental conditions chosen for this study energy dissipation rates would typically have to be less than 80 W/kg and the suspension mass concentration less than 2.5%. When compared with water, a reduction in turbulence levels at both the impeller zone and a remote zone in the stirred vessel was observed for fibre mass concentrations as low as 0.5%. The turbulence decreased as the suspension mass concentration was increased. This decrease is attributed to energy dissipation by friction at fibre‐fibre contact points as the fibres move relative to one another in the flow. This sink removes energy from the turbulence cascade which never shows up as small‐scale fluid deformations leading to better m