Thermal expansion of Cr-doped (0, 2.30, 4.74, 6.97, and 9.17 wt% Cr2O3) mullites derived from single-phase precursors was studied by in situ (480–940 °C) powder X-ray diffraction. Chromium doping caused an increase in unit-cell parameters. The strongest expansion due to Cr incorporation into mullite was noticed along the c-axis, followed by a and b. In the temperature range from 480 to 940 °C, the lattice parameters expanded almost linearly for all prepared mullite samples. Thermal expansion of Cr-doped mullite was smaller than of pure mullite. Both undoped and Cr-doped mullites displayed strongest thermal expansion along the b-axis, followed by c and a. Coefficients of thermal expansion along the a, b, and c-axes displayed minima for mullite doped with 6.97 wt% Cr2O3.