The main limitation of oxide ceramic matrix composites (Ox-CMCs) is related to their thermal stability. At temperatures above 1000°C, fiber grain growth and matrix densification lead to loss of strength and embrittlement. To improve the thermal stability of Ox-CMCs, the use of matrix doping is analyzed in this study. Minicomposites containing Nextel 610 fibers and alumina matrix with and without 450 ppm of MgO doping were evaluated before and after different thermal exposures to 1300°C and 1400°C for 2 h. Both minicomposite types have very similar as-produced properties. Nevertheless, MgO-doped minicomposites show much lower fiber grain growth during the subsequent thermal exposures. The differences in microstructural evolution are associated with elemental diffusion between fiber and matrix at high temperatures. In this sense, the MgO doping from the matrix can diffuse to the fibers and also influence fiber grain growth. As a result, MgO-doped minicomposites show much higher strength retention than non-doped mminicomposites after the thermal exposures. Hence, doping the composite matrix can increase the overall thermal stability of Ox-CMCs.