The high-temperature (HT) behavior of BaAs2Zn2O8∙H2O was studied by in situ single-crystal X-ray diffraction (SCXRD) and hot stage Raman spectroscopy (HTRS) up to dehydration and the associated phase transition. During heating, the studied compound undergoes the dehydration process with the formation of BaAs2Zn2O8, which is stable up to at least 525 °C. The evolution of the fourteen main Raman bands was traced during heating. The abrupt shift of all Raman bands in the 70–1100 cm–1 spectral region was detected at 150 °C, whereas in the spectral region 3000–3600 cm–1 all the bands disappeared, which confirms the dehydration process of BaAs2Zn2O8∙H2O. The transition from BaAs2Zn2O8∙H2O to BaAs2Zn2O8 is accompanied by symmetry increasing from P21 to P21/c with the preservation of the framework topology. Depending on the research method, the temperature of the phase transition is 150 °C (HTRS) or 300 °C (HT SCXRD). According to the HT SCXRD data, in the temperature range 25–300 °C the studied compound demonstrates anisotropic thermal expansion (αmax/αmin = 9.4), which is explained by flexible crankshaft chains of TO4 (T = As, Zn) tetrahedra. Additionally, we discussed some crystal-chemical aspects of minerals with both (ZnOn) and (AsOm) polyhedra (n = 4, 5, 6; m = 3, 4) as main structural units.