Two new sodium zinc antimonides NaZn4Sb3 and HT-Na1-xZn4-ySb3 were synthesized by using reactive sodium hydride, NaH, as a precursor. The hydride route provides uniform mixing and comprehensive control over the composition, facilitating fast reactions and high-purity samples, whereas traditional synthesis using sodium metal results in inhomogeneous samples with a significant fraction of the more stable NaZnSb compound. NaZn4Sb3 crystalizes in the hexagonal P63/mmc space group (No. 194, Z = 2, a = 4.43579(4) Å, c = 23.41553(9) Å), and is stable upon heating in vacuum up to 736 K. The layered crystal structure of NaZn4Sb3 is related to the structure of the well-studied thermoelectric antimonides AeZn2Sb2 (Ae = Ca, Sr, Eu). Upon heating in vacuum NaZn4Sb3 transforms to HT-Na1-xZn4-ySb3 (x = 0.047(3), y = 0.135(1)) due to partial Na/Zn evaporation/elimination, as was determined from hightemperature in-situ synchrotron powder X-ray diffraction. HT-Na1-xZn4-ySb3 has a complex monoclinic structure with considerable degrees of structural disorder (P21/c (No. 14, Z = 32), a = 19.5366(7) Å, b = 14.7410(5) Å, c = 20.7808(7) Å, β = 90.317(2)°) and is stable exclusively in a narrow temperature range of 736 − 885 K. Further heating of HT-Na1-xZn4-ySb3 leads to a reversible transformation to NaZnSb above 883 K. Both compounds exhibit similarly low thermal conductivity at room temperature (0.9 W•m −1 K −1 ) and positive Seebeck coefficients (38-52 µV/K) indicative of holes as the main charge carriers. However, resistivities of the two phases differ by two orders of magnitude.Here, we have explored the ternary Na-Zn-Sb system and discovered two new compositionally similar, but structurally different ternary antimonides, both are featuring new structure types. Using the fast hydride route coupled with in-situ high-temperature powder X-ray diffraction experiments, compositional and temperature screening allowed for synthesis of two new ternary phases: NaZn4Sb3 phase and what at first appeared to be its polymorph, but in fact it is a different compound with slightly Na/Zn depleted composition HT-Na1-xZn4-ySb3, stable in narrow temperature range. The hydride route yields single phase samples of both antimonides, allowing for the experimental access to their transport properties. The crystal structures, synthesis, structural transformations, and transport properties of the NaZn4Sb3 and HT-Na1-xZn4-ySb3 are discussed herein.