Accurate values of both the reaeration coefficient (K2) and the longitudinal dispersion coefficient (Dx) are important for reliable water quality modelling. The aim of this study was to quantify both K2 and Dx for a test reach of the River Lagan in Northern Ireland, using stable Kr and Rhodamine WT tracers simultaneously. A simple spreadsheet model combined with non-linear regression was used both to fit the 1-D Advection Dispersion Equation (ADE) to the tracer breakthrough curves and optimise parameters of the ADE. Under flows of 3.2–7.5 m3/s, Dx was 7.9–17.0 m2/s and K2 at 20◦C was found to be 4–27 d−1. This study strengthens the case for use of Kr and RWT as environmentally-friendly tracers to determine not just K2 but Dx too.