Brilliant Blue is often used to trace water movement in soils. Its concentration in soil samples is usually determined by extraction – a laborious procedure with varying accuracy. We show that Brilliant Blue can be estimated directly by visible diffuse reflectance spectroscopy. We build a PLSR (partial least squares regression) model for the concentration range of 0.1 to 15 mg Brilliant Blue per g soil with an RMSE (root mean square error of prediction) of 1 mg g-1 and an R2 adj of 0.9. However, as the method is based on visible spectra, prediction accuracy can be altered by variations in soil colour between calibration and prediction data sets.