SUMMARY. 1. Dominant chemical gradients in the peats of a flood-plain fen in Broadland, Norfolk were poor-fen (oligotrophic)  rich-fen (minerotrophic) and freshwater  brackish water. These gradients were to some extent obscured by a complex of factors governing the concentrations of ions in the peats and peat waters.

2. The peats were almost totally organic and cation exchange capacities (CEC) varied with their macrofossil composition. Cladium mariscits L. peats had higher CEC than Phragmites communis Trin. peats of similar bulk density.

3. Amounts of dissolved and extractable N and P were very low. but total amounts of N and P were high and closely related to bulk density.

4. Large seasonal variation in concentrations of ions in the peat waters was due to dilution at times of flooding. There was strong evidence that little river water penetrated directly across the study area, contrary to classical descriptions of flood-plain mires.

5. Amplitude of fluctuations in concentration varied between study sites although the pattern of change was similar. Fluctuations were small in poor-fen. Sphagnum dominated sites.

6. An incursion of brackish water up river due to unusually high tides was observed, but it is likely that brackish conditions in the fens are caused by release of ions from underlying estuarine clays.