The present work was carried out to determine the effect of different factors on the sintering of alumina using volume shrinkage as the sintering criteria, and to examine the mechanism of sintering over the temperature range 1200°C to 1950°C. Up to 1450°C this work agrees with the accepted theory of sintering by volume diffusion (shrinkage proportional to time to the power of two fifths), whereas, above 1450°C this work was found to differ from. the present-day theories of other workers, by giving a time relationship of a one fifth power. An explanation has been formulated using the concept of an exponential rise in the equilibrium concentration of vacancies present in the crystal lattice affecting the concentration gradient of defects from the particle junctions to the surface. The effect of preferring at temperatures below that at which sintering begins has been known empirically for some years, The present work explains this behaviour by considering the diffusion of atoms along the surface of the particles to give the formation of necks prior to shrinkage. Sintering from 1770°C to 1950°C was performed in vacuo and the temperature range was extended below 1770°C to 1600°C to compare the results with those obtained in air. The disparity observed has been explained on the basis of evaporation of high-energy areas giving a lower driving force for sintering and subsequent lowering of the shrinkage values compared with sintering in air A comparative examination of the effect of particle size and shape indicates the importance of the initial particle size as a control of the degree of sintering, The effect of additions of 1%, 5%, 10% liquid using sodium silicate as a liquid former, shows that an appreciable amount of liquid is not necessary for liquid-phase sintering, and that for the solution-precipitation stage only enough liquid need be present to form a lens at the particle junctions, With 1% liquid addition no liquid-phase sintering occurs, but enhanced solid state sintering is observed due to formation of b-alumina Na2O 11 A1203.