In this study, we apply ultrasound velocity profiling (UVP) for obtaining velocity data inside black liquor nozzles. We demonstrate the capability of this technique for online measurement of black liquor flow properties and viscosity behavior. The experiments were carried out on an operating recovery boiler where the black liquor from pulping softwood was taken directly from the boiler liquor system and sprayed through a customized splash-plate nozzle. Application: Inline rheology measurement in black liquor nozzles can lead to improved efficiency in the spraying process control. Measurements can also give insight to processes taking place in nozzle flow that have not been possible to measure before.