AbstractViscous flow is the main sintering mechanism in glass matrix composites (GMCs) and ceramic glazes and bodies. The microstructural changes that occur in the sintering of these materials include continuous development of apparent and closed pore shape, size, and volume and of the relationship between apparent and total porosity, which determines the material’s final porosity. Final porosity is probably one of the major characteristics of these materials, as it determines many resulting material performance properties. This study examines the microstructural development and sintering of glass–zircon composites with different volume fractions of rigid inclusions: f=0, 0.05, 0.11, 0.17, 0.32, 0.43, 0.53, and 0.65. The test composite sintering curves and the variation of the above characteristics with temperature and zircon grain and crystallite size were experimentally determined. The parameters relating to zircon particle connectivity were theoretically estimated with a view to interpreting the sintering of the composites via particle rearrangement by viscous flow, using the percolation theory. The zircon solution–reprecipitation mechanism was verified to only occur at temperatures above 950 °C.