We have investigated the system ZnSnSb 2 in the course of our attempts to modify thermoelectric Zn-Sb frameworks. ZnSnSb 2 is only accessible when employing Sn as reactive flux in the synthesis. The material shows an order-disorder transition in the temperature interval between 225 and 240 °C and decomposes peritectically at about 360 °C. The high-temperature form of ZnSnSb 2 adopts the Zn/Sn disordered cubic sphalerite-type structure. Electron microscopy investigations reveal that samples quenched from 350 °C already contain domains of the low-temperature form, which has the Zn/Sn ordered tetragonal chalcopyrite structure. The c/a ratio of the tetragonal structure is, within experimental errors, identical to the ideal value 2. This gives rise to intricate microtwinning in the low-temperature chalcopyrite form of ZnSnSb 2 as obtained in samples quenched from 250 °C. First principles electronic structure calculations demonstrate that the tetragonal low-temperature form of ZnSnSb 2 has a narrow band gap of about 0.2 eV. This is in agreement with the semimetallic behavior of the material found from resistivity measurement. The shape of the electronic density of states for ZnSnSb 2 is similar to thermoelectric binary Zn-Sb frameworks. However, the thermopower of ZnSnSb 2 is rather low with room-temperature values ranging from 10 to 30 µV/K.