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beta-Zn4Sb3 is an outstanding thermoelectric material mainly due to its extraordinarily low thermal conductivity, which is similar to that of glasses. Recently it was proposed that interstitial Zn atoms are responsible for this peculiar behavior. Here we report on the crystal and electronic stucture of the low-temperature polymorph alpha-Zn4Sb3. During the reversible phase transition the intricate disorder in beta-Zn4Sb3 disappears, and all Zn atoms localize completely. The electronic structure of alpha-Zn4Sb3 corresponds to that of a narrow-gap semiconductor.