High- temperature electron microscopy was used to follow how the structure of two specimens of three-dimensionally ordered macroporous ( 3DOM) materials, also known as inverse opals, and one specimen of a precursor to a 3DOM material changed with temperature. The change in grain size with temperature of 3DOM cobalt and 3DOM iron oxide ( as magnetite) was monitored in situ in the TEM by heating in stages to 900 and 1,000 degrees C, respectively. The two materials studied by TEM showed contrasting grain growth behavior. For 3DOM cobalt, carbon surrounding the nanometer- size grains led to slower grain growth in thinner sample areas than in areas in closer contact with other grains; a bimodal grain- size distribution was observed after heating above 700 degrees C for 90 min. The grains of the 3DOM iron oxide had no carbon coating and coarsened more evenly to give a unimodal size distribution. Line scans from selected- area diffraction ( SAD) patterns were used for phase analysis and showed that traces of cobalt oxide present in the 3DOM cobalt sample at room temperature disappeared when the sample was heated above 500 degrees C. The transformation of a 3DOM precursor material, nickel( II) oxalate - polystyrene ( PS) latex composites, was followed in situ by variable- temperature environmental scanning electron microscopy ( ESEM) from room temperature to ca. 700 degrees C in 0.5 - 0.7 kPa O-2. The ESEM examination of the 3DOM precursors permitted real- time observation of the polymer template decomposition and the shrinkage that occurs upon calcination of these precursor materials.