Kinetics and mechanism of sintering in a model borosilicate glass (BSG) + alumina composite system have been investigated isothermally at 600 to 1000 °C. A faster and greater densification is observed with higher sintering temperature, greater BSG content, and larger alumina particle size. Using measurable densification kinetics, it is found that the activation energy of densification (Q) increases continuously with increasing BSG content, from predominant <110 kJ/mol for BSG < 40 vol.% to 245 kJ/mol for BSG ≥ 90 vol.%. The rate-limiting step during densification is identified to be diffusion of alkali ions in BSG when BSG < 40 vol.% (Q = 110–170 kJ/mol), diffusion of both alkali and aluminum ions when BSG = 40–60 vol.% (Q = 110–170 kJ/mol), diffusion of aluminum ion in BSG when BSG = 60–80 vol.% (Q = 170 kJ/mol), and viscous flow of BSG when BSG ≥ 90 vol.% (Q = 245 kJ/mol). These observations are attributed to a chemical reaction taking place at the interface of alumina/BSG, resulting in a reaction layer adjacent to alumina. Since the composition of the reaction layer is known to be rich in aluminum and alkali ions and poor in silicon, the alkali ions content in BSG is continuously decreased during sintering. Accordingly, when the BSG content in the mixture is low, the resultant loss of alkali ions from BSG causes a rise in viscosity of BSG, thus slowing down the densification kinetics and changing the densification mechanism. As the amount of BSG is increased to ≥90 vol.% at the expense of alumina, the reaction as described above becomes less dominant and the sintering proceeds by viscous flow of glass.