The yield stress and flow behaviour of ultrafine Mallee biochar-water slurries were evaluated as a function of particle size distribution, solid concentration and additives including carboxymethylcellulose sodium salt (CMC) and poly(budadiene-maleic acid) sodium salt (BMA). The Mallee biochar was made from Oil Mallee chips carbonised in the absence of air at 750 degrees C in an indirectly-fired kiln with a solid residence time of ca. 30 min. The biochar-water slurries were prepared, with and without additives, by (I) dry-milling the biochar for different times and then mixing the resulting powders with water and (2) by wet-milling the biochar also for different times. The milling, dry or wet over different times resulted in different particle size distributions, the effect which on the rheological properties on the slurries was also investigated. The slurries displayed highly shear-thinning flow behaviour, typical of a floc-culated dispersion. It was found that the yield stress decreased with increasing particle size and the relationship can be represented by a power law model. The yield stress showed a dependence on D-50 (median diameter) or D-4,D-3 (volume average diameter) to the power of -0.8. In the presence of 1 and 5 dwb% BMA, the dependence on D-4,D-3 was to the power of -2 and -3, respectively. A power law model was also shown to describe the relationship between the yield stress and the solid volume fraction. The power law index ranged from 11.3 to 12.3 for slurries prepared from wet-milled biochar powders and from 14 to 18 for slurries prepared from dry-milled biochar powders. The highly charged BMA additive did not completely disperse the Mallee biochar slurries however it did reduce the yield stress and viscosity of the slurries significantly. Zeta potential data also strongly indicate the occurrence of BMA and CMC adsorption on the char particles. However, the CMC additive did not produce the significant reduction in the yield stress as expected. (C) 2012 Elsevier Ltd. All rights reserved.