An important by-product of the kraft pulping process, black liquor is currently used in most pulp mills to produce steam in a recovery boiler. Two important black liquor characteristics are viscosity and burning (firing) characteristics, which determine the operating efficiency of the recovery boiler. Kraft black liquor is a complicated material, with physical properties varying greatly depending on starting material (chip supply), pulping process (chemicals), solids content and temperature. The effects of solids content and temperature on black liquor and black liquor viscosity have been extensively studied. However, considerably less work has been done to examine the effect of caustic addition on black liquor viscosity and firing characteristics. In the present study, the effective alkali content (EAr) of the black liquor was variedby a controlled addition of caustic (NaOH) solution. Several mill samples of varying history and % solids were studied using a rheometer to measure viscosity and thermal characterization (DSC and TGA) to determine burning characteristics. The results indicate that the addition of caustic to black liquor significantly reduces viscosity at all solids contents and temperatures up to a maximum EAr level of approximately 4% where the viscosity then either levels off or starts to increase depending on the type of black liquor. The results show that caustic addition does not adversely effect burning characteristics as determined by the energy required to evaporate the sample water prior to combustion and the subsequent thermal degradation point of the organic matter. The results of the present studies support the use of caustic addition to control black liquor viscosity in the evaporator and recovery boiler processes. The "optimum" EAr level to obtain the lowest viscosity must be determined for the specific process liquor