Abstract: A method and apparatus are provided for analyzing the elemental constituents of fluid streams in the cellulose pulp industry, such as liquid with dissolved solids for treatment of cellulose pulp or from the treatment of cellulose pulp, or liquid slurries including comminuted cellulose material. Continuous on-line elemental analyses are made of one or more such fluid streams, and at least the partial elemental make-up (either material, or of elements added for marking) of the streams determined. Then one or more processes for acting on the fluid streams are controlled to optimize the processes as a result of the analyses made. The invention is particularly useful in the continuous on-line analysis of black liquor utilizing prompt gamma neutron activation analysis, and controlling the operation of an evaporator or recovery boiler.