After the close of the Callide Oxyfuel Project, mature deposit samples were collected from the primary and secondary superheater tubes. These samples were studied with the aim of providing a description of deposits from an industry scale oxy-fuel plant and to gain some understanding of the deposition mechanism involved in their formation. The deposits formed over a 3 year operating period, with both air and (more extensive) oxy-firing, including many air-to-oxy transitions, boiler starts and trips, and fired predominantly by a coal with Fe2O3 in ash analysis of about 10%. The mineral character of the coal is relatively simple with silicon and aluminum clays and silica and iron-containing siderite dominant. This character allows for the plotting of chemical analysis results in triangular phase diagrams used to interpret scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis, with iron being associated with the slagging mechanisms. The deposits were unusual in that layers of iron-rich particles and alumina silicates formed 10-50 mu m from the interface with the metal tubes, running parallel to the direction of deposit growth and normal to the tube surface, showing that deposition of each layer occurs concurrently.