Petrographic and chemical studies were made on the black cherts of the San Antonio formation (eastern Venezuela) and of the Ftanita del Tachira member (western Venezuela), both of Upper Cretaceous age. The AI2O3, Fe2 (total), MnO, CaO, MgO, K2O and Ti2 contents were analyzed by X-ray fluorescence; S12 contents were analyzed by X-ray fluorescence and gravimetric methods. Na2 was measured using atomic absorption techniques. Gammaray spectrometric methods were used for the determination of U and Th in the rocks from eastern Venezuela. The type of carbonate present was identified by staining the thin sections with K ferricyanide and Alizarin red S. At the microscope, the cherts can be divided into three broad types: (a) pure cherts, with microquartz in the matrix and few microfossils; (b) shaly cherts, with variable amounts of calcite, clay minerals and microfossils; (c) calcareous cherts, with abundant microfauna and calcareous matrix. A fourth type of rock present in the cherts is represented by the calcareous nodules. Combined stratigraphic, lithologic and petrographic data show that the sediments from which the cherts of eastern and western Venezuela derived, were deposited in similar euxinic environments. The Fe/Mn ratios suggest that the sedimentation happened in deep waters. The low Th/U ratios of the rocks from eastern Venezuela are consistent with low rates of sedimentation. The low TiOg/AO and FegO-AlO ratios exclude that the source of Si2 was basic volcanism. The low Th contents in the San Antonio formation suggest that acidic volcanism was not an important source of silica. The presence of partially preserved radiolarians supports the biogenic origin of silica for both cherts. A simplified model of sedimentation in euxinic basins, which explains the alternation of deposition of siliceous and calcareous sediments, is proposed. According to this model, the pH is the factor controlling the solution of the calcareous tests and the consequent concentration of biogenic silica. The deposition of the more argillaceous sediments from which the shaly cherts originated, was probably related to the flushing of the basins by oxygenated currents. The pure cherts were formed by solution of opaline tests of radiolarians and redeposition of the Si2 as void filling, before compaction. The silicification of the matrix and of the calcareous tests happened probably in a later stage. Plots of Si2 contents vs Si2/Al2 ratios and of CaO contents vs SiOg/A^O-j ratios show that the chemical composition is consistent with the formation of these cherts by introduction of Si2 to an original siliceous ooze. The formation of the calcareous cherts In which many calcareous tests are preserved, and the variable proportions of calcite and silica filling the tests, must be related to different production of NH^ and COg by bacterial decay in the early diagenesls. Plots of Si2 contents vs Si2/Al2 ratios and CaO contents vs SlOg/AO ratios show that the derivation of these cherts from a globigerinld ooze, by introduction of silica, would agree with their chemical composition. The calcareous nodules were formed during the early diagenesis of the sediments, before compaction. The replacements of S12 by rhombs of calcite, present in the calcareous cherts, happened after lithiflcation of the sediments.