Mixing of the reburn fuel with primary combustion-zone products is one of the most important parameters affecting NOx reduction. The mixing can be controlled by reburn jet parameters. The objective of the study is twofold: to assess the ability of Computational Fluid Dynamics (CFD) for predicting properties of the jets, and to develop a methodology for scaling the results obtained for isothermal flows into combusting flows in boilers and municipal-waste incinerators. To this end the fluid-flow experiments are performed using the IFRF stand for simulating flows in boilers and incinerators, The Laser Doppler technique is used to measure 3-dimensional flows, and the Laser Sheet technique (Mie scattering) is used to visualise the reburn jets, their trajectories and mixing. The CFD-computed time-mean 9-dimensional flow fields agreed well with the measured flow patterns. The predicted maximum penetration depth differed by 5-12% from the experimentally observed trajectories. Each of the eight reburn jets responded to the local flow conditions prevailing upstream of the injection position. Numerical simulations of the reburn-jet penetration depth and mixing required a good knowledge of the overall flow pattern within the boiler. This paper contains correlations for estimating the jet-penetration depth under isothermal and combustion conditions. It has been demonstrated that engineering correlations describing the jet trajectories in a cross-flow could not be applied to the boiler flow considered.