Wall-to-bed heat transfer coefficients to pulp mill Kraft black liquor in a nominal 25-mm tubular, liquid?solid fluidized bed have been determined. The fluidizing particles were stainless steel balls of 4.0, 4.76, and 6.35 mm diameter. The heat transfer coefficients were determined for conditions where porosity, bulk temperature, and liquor concentration were varied. The best correlation was obtained when the data were split into two groups of low and high liquor concentrations, i.e., 30?40% and 50?65% total dissolved solids. The data were also compared with the predictions obtained from 26 published correlations, of which the best 10 are presented and discussed in this article. The results of this investigation are relevant to other fluidized bed systems with high liquid viscosity, such as sugar concentrations, hydrocarbons, and polymers.