Thinly interbedded estuarine sandstones and mudstones in the middle Eocene Wittering Formation (Bracklesham Group), Isle of Wight, contain authigenic clinoptilolite, opal-CT, and possibly authigenic smectite over a thickness of 8.5 m. The clinoptilolite and opal-CT cements have resulted in these beds being the only ones tightly cemented in the Wittering Formation, the rest being soft sands and muds. The unit contains abundant siliceous sponge spicules, many of which are preserved as voids and were the likely source of the opal-CT. The presence of an organic-rich, rooted paleosol (histosol) immediately overlying the unit is believed to have contributed significantly to the early diagenetic history of the estuarine sediments, in particular the formation of zeolite. Although the pH and dissolved silica of unmodified seawater are too low for zeolite formation, high concentrations of dissolved silica are believed to have resulted from dissolution of siliceous sponge spicules (opal-A) and formation of silica-organic acid complexes. Organic-acid dissolution of feldspars (absent from the zeolite-bearing interval) is the most probable source of alumina and cations for zeolite formation. REE data show that volcanic ash was never present and cannot therefore have been the source of the zeolite. This investigation therefore demonstrates a novel route by which the pore-fluid chemistry required for zeolite formation can be achieved.