Although alkali-alkali earth carbonates have not been reported from mantle-derived xenoliths, these carbonates may have a substantial role in mantle metasomatic processes through lowering melting temperatures. On the Na2Mg(CO3)2–K2Mg(CO3)2 join only the Na-end-member eitelite (R3̄ space group), was reported in nature. The K-end-member (R3̄m) readily hydrates even at low temperatures, therefore, only baylissite, K2Mg(CO3)2·4H2O, has been observed. Because of the role of (K,Na)Mg-double carbonates in mantle metasomatism, we performed high P-T experiments on K2Mg(CO3)2, (K1.1Na0.9)2Mg(CO3)2, and Na2Mg(CO3)2. Structure refinements were done upon compression of single crystals from 0 to 9 GPa at ambient temperature employing synchrotron radiation. Fitting the compression data to the second-order Birch-Murnaghan EoS resulted in V0 = 396.2(4), 381.2(5), and 347.1(3) Å3 and K0 = 57.0(10), 54.9(13), and 68.6(13) GPa for K2Mg(CO3)2, (K1.1Na0.9)2Mg(CO3)2, and Na2Mg(CO3)2, respectively. These compressibilities are lower than those of magnesite and dolomite. The KMg-double carbonate transforms into a monoclinic polymorph at 8.05 GPa; the high-P phase is 1% denser than the low-P polymorph. The NaMg-double carbonate has a phase transition at ~14 GPa, but poor recrystallization has prevented structure refinement. The parameters for a V-T EoS were collected at 25–600 °C and ambient pressure and are α0 = 14.31(5) × 10−5 K−1 and 16.73(11) × 10−5 K−1 for K2Mg(CO3)2 and Na2Mg(CO3)2, respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a- and c-axis: α0(a) = 2.84(6) × 10−5 and 4.78(5) × 10−5 K−1 and α0(c) = 10.47(11) × 10−5 and 8.72(5) × 10−5 K−1 for K2Mg(CO3)2 and Na2Mg(CO3)2, respectively.