The electrochemical reaction of FeSb2 with Na is reported for the first time. The first discharge (sodiation) potential profile of FeSb2 is characterized by a gentle slope centered at 0.25 V. During charge (Na removal) and the subsequent discharge, the main reaction takes place near 0.7 V and 0.4 V, respectively. The reversible storage capacity amounts to 360 mA h g(-1), which is smaller than the theoretical value of 537 mA h g(-1). The reaction, studied by ex situ and in situ X-ray diffraction, is found to proceed by the consumption of crystalline FeSb2 to form an amorphous phase. Upon further sodiation, the formation of nanocrystalline Na3Sb domains is evidenced. During desodiation, Na3Sb domains convert into an amorphous phase. The chemical environment of Fe, probed by (57)Fe Mössbauer spectroscopy, undergoes significant changes during the reaction. During sodiation, the well-resolved doublet of FeSb2 with an isomer shift around 0.45 mm s(-1) and a quadrupole splitting of 1.26 mm s(-1) is gradually converted into a doublet line centered at about 0.15 mm s(-1) along with a singlet line around 0 mm s(-1). The former signal results from the formation of a Fe-rich FexSb alloy with an estimated composition of 'Fe4Sb' while the latter signal corresponds to superparamagnetic Fe due to the formation of nanosized pure Fe domains. Interestingly the signal of 'Fe4Sb' remains unaltered during desodiation. This mechanism is substantially different than that observed during the reaction with Li. The irreversible formation of a Fe-rich 'Fe4Sb' alloy and the absence of full desodiation of Sb domains explain the lower than theoretical practical storage capacity.