Due to the dynamic interactions of phosphorous species in natural systems, concentrations of individual species may change when samples are stored. Accurate analysis is, therefore, difficult unless measurements are made in situ. The technique of diffusive gradients in thin-films (DGT) has been developed for the in situ measurement of reactive phosphorus species in natural waters, sediments and potentially soils. Phosphorus species diffuse through a layer of polyacrylamide gel and then bind to ferrihydrite embedded in a further layer of gel. The mass of phosphorus in the ferrihydrite after a known deployment time is measured colorimetrically after acid elution. Concentration in the solution is then calculated using Fick's first law of diffusion. The diffusion coefficient of orthophosphate in the gel was independently measured to be 6.05×10−6 cm2 s−1 at 25°C which is 71% of the value in water. Using this value of the diffusion coefficient, DGT could be used to measure phosphate in solutions accurately without calibration. The preparation of the ferrihydrite is critical as irreproducible results were obtained when alternative iron oxide preparations were used. DGT measurements of reactive phosphorus species in situ in a eutrophic pond agreed well with mean replicate filterable reactive phosphorus (FRP) measurements made on a time series of collected water samples. For deployments of 24 h and 1 week, respectively, the limits of detection should be 0.07 μg l−1 of P and 0.01 μg l−1 of P. DGT was used to measure reactive phosphorus species in pore waters of a freshwater sediment at a spatial resolution of 1 mm.