Hyporheic and parafluvial flows between streams and the underlying streambed, or adjacent alluvium, are important drivers of biogeochemical cycling in streams. Here we present a new method for characterising this exchange in a losing stream based on longitudinal stream radon activities. A mass balance approach is used to constrain the radon influx into the stream and estimate exchange parameters: flux, residence time and exchange zone thickness. A net radon flux into the stream of 5.4×104Bqm-1d-1 is required to balance radon losses to groundwater recharge, gas transfer and radioactive decay. Given the radon production rate of the sediments (1.3±0.7BqL-1d-1), the minimum volume of alluvium flushed by either hyporheic or parafluvial exchange is 168m3 per m length of stream. Based on the stream width, depth of alluvial sediments and porosity, this implies that the exchange zone extends beneath the stream and an additional 11m either side. The results of this new method are compared to two existing methods; streambed radon disequilibrium and transient storage modelling of breakthrough curves of an injected tracer. The stream radon mass balance provides a relatively simple means of estimating hyporheic (and parafluvial) exchange over tens to hundreds of kilometres of stream. Concurrent application of the stream radon method, transient storage modelling of injected tracer breakthrough curves and hydraulic methods is recommended to capture the full spectrum of hyporheic exchange in losing streams.