The phase evolution and thermal expansivity of synthetic orthorhombic BaCO3 (α-BaCO3) and SrCO3 (α-SrCO3) were studied up to 1413 K using high-temperature powder X-ray diffraction. The α-BaCO3 phase transformed into a trigonal β phase at 1073–1093 K with an increase in molar volume by 2.7%, and further to a cubic γ phase at 1233–1253 K with 0.2% increase in molar volume. The α-SrCO3 phase transformed into a trigonal β phase at ~1173 K. The fitted parameters for volume thermal expansion coefficients (αV(T) = a0 (10−5 K−1) + a1(10−8 K−2)T) are a0 = 4.0(4), a1 = 4.0(6) for α-SrCO3; a0 = 4.4(5), a1 = 3.9(7) for α-BaCO3; a0 = 6.5(2) and a0 = 6.8(1) for the β and γ phase of BaCO3, respectively. The β-BaCO3 phase exhibits negative expansion along the a-axis, which may help to design new materials with tunable thermal behavior when it is mixed with materials with positive thermal expansion. The aragonite-group carbonates (CaCO3, SrCO3, and BaCO3) have similar bulk thermal expansivity, but their anisotropy and linear expansivities vary with the size of the cation.