Abstract Phosphorus fractionation studies were undertaken on seven UK tufas: three modern, and four old. The phosphorus in the carbonate fraction of the tufa averaged 19% of the total phosphorus in the modern material, and from 48–64% in the 4000 year-old deposits and the increase attributed to the mineralization of the contained organic matter. Two further phosphorus fractions contained significant amounts of P. The dithionite fraction ranged from 10–27% of the total, and most of this fraction was probably associated with detrital iron minerals. The alkali-soluble fraction which removed most of the organically-bound P was highest in the modern bryophyte tufas. Total P levels in the tufas ranged from 33–119 ppm. These are low values for stream sediments. Phosphorus uptake rates into tufa were estimated for these sites. They were less than 5% of the incoming P and deemed to have a negligible effect on the aquatic biota.