The polymorphic intermetallic compound Zn5Sb4In2−δ (δ = 0.15(3)) shows promising thermoelectric properties at low temperatures, approaching a figure of merit ZT of 0.3 at 300 K. However, thermopower and electrical resistivity changes discontinuously at around 220 K. Measurement of the specific heat locates the previously unknown temperature of the order-disorder phase transition at around 180 K. Investigation of the charge carrier concentration and mobility by Hall measurements and infrared reflection spectroscopy indicate a mixed conduction behavior and the activation of charge carriers at temperatures above 220 K. Zn5Sb4In2−δ has a low thermal stability, and at temperatures above 470 K samples decompose into a mixture of Zn, InSb, and Zn4Sb3.