The activities of Nd in the Nd-Fe alloys were determined by double Knudsen cell mass spectrometry. The activities of Nd in Nd-Fe alloys with Nd concentration from 8.5 to 93 at% were determined at 1373-1523K, measuring the ion current of vapor of Nd in equilibrium with melts, Nd2Fe17 + gamma Fe, Nd2Fe17 + liquid, and gamma Fe + liquid. The standard Gibbs energy of formation of Nd2Fe17 was calculated from the activities of Nd obtained from the measurement, as follows: 2Nd(I) + 17Fe(gamma) = Delta G(f)(o)Nd(2)Fe(17)(s), = -238.000 + 130T +/- (21, 000)J/mol (1373-1473K). The activities of Fe in Nd-Fe alloys were calculated based on the Gibbs-Duhem equation. (C) 2011 Elsevier B.V. All rights reserved.