This paper presents an application of the inverse Nyquist array (INA) technique to the design of a multivariable controller for a boiler subsystem. The boiler is a 1.6 MVV water boiler using solid fuel. The subsystem studied consists of the boiler underpressure and the flue-gas oxygen content. First a simple transfer function model based on step response measurements is constructed. The subsystem model is analysed and the interactions are decreased with a constant compensator using Rosenbroek's INA method. Finally, scalar PI controllers are tuned for the approximately decoupled system. A brief description of the microcomputer control system is given. The control system and the designed multivariable controller have worked well since their installation in February 1983.