Two types of authigenic zeolites from Late Pleistocene, hemipelagic sediments of the South Atlantic were examined by XRD- and SEM-analysis. The XRD-characteristics and the thermal behaviour of the zeolites suggest that type 1 is composed of clinoptilolite. Type 2 was identified as mordenite, based on morphological features and qualitative elemental composition. Their authigenic nature is suggested from the shape of the crystals and their growth on and within foraminifera tests. Volcanic ash material, acting as precursor minerals to zeolite formation could not be found, nor were the zeolites directly related to siliceous remains. An authigenesis from pore waters must be assumed. Mineral stabilities for amorphous SiO2 and zeolites were calculated with the computer program solmineq88. Interstitial waters proved to be oversaturated throughout the core in respect to all zeolites considered. Biogenic opal represents the limiting solid phase for Si-concentrations in pore waters. Therefore, the dissolution of opal is regarded as the main source of Si for interstitial waters. Below 9 m core depth, where opal is only present in traces, detrital feldspars and the zeolites themselves are affected by dissolution. This may indicate the metastable nature of the zeolites, also corroborated by the presence of two, almost isochemical members of the same zeolite group. © 1993.