Siliceous sinter, loose sediments, and suspended flocs in Champagne Pool, an anoxic hot (75 °C) spring at Waiotapu, New Zealand, are composed of opaline silica and metal-rich sulphides that contain many well-preserved, mineralized microbes. Detailed analysis by transmission electron microscopy and energy dispersive spectrometry has shown that bacterial cell wall and capsular material is preserved by the immobilization of high levels of As, Sb and S in the organic matrix. Calculation of the probable metal species in the spring water suggests that arsenic and antimony are present in solution as negative and neutrally charged sulphide or hydroxide complexes (such as HAs2S4−, H3AsO3 and HSb2S4−). The early adsorption of these complexes onto reactive groups on the bacterial surface may be paramount in the excellent preservation of cell morphology. As biomineralization progresses, biomineral composition commonly becomes dominated by the precipitation of a supersaturated Al-rich amorphous silica phase. Biomineralization commonly preserves S-layers, an ordered mosaic of proteins on the outer surface of the cell wall. These are the finest ultrastructure details thus far found in microbes preserved by hydrothermal mineralization, and can be used as an aid to identify microfossils. The S-layers preserved here probably belong to Clostridium thermohydrosulfuricum or Desulfotomaculum nigrifacans.