The two title compounds were prepared by direct reactions of the corresponding elements at high temperature, and their structures were determined from single-crystal X-ray diffraction data. The structure of K38Nb7As24 (orthorhombic; Cmcm; Z = 4; a = 10.4974(6), b = 23.915(2), c = 36.046(2) Angstrom) comprises isolated tetrahedra of NbAs4 and two types of dimers of edge-sharing tetrahedra: dimers containing only Nb-V, [Nb(V)2As(6)](8-), and mixed-valence dimers with both Nb-IV and Nb-V, [(NbNbAs6)-Nb-IV-As-V](9-). The structure of Cs9Nb2As6 (orthorhombic; Pbca; Z = 8; a = 17.5848(7), b = 16.940(2), c = 18.183(4) Angstrom) contains only the latter climers. Magnetic measurements showed Curie-Weiss paramagnetic behavior for both compounds consistent with one unpaired electron/mixed-valence dimer. Cs9Nb2As6 exhibits also an antiferromagnetic transition at about 36 K. The two compounds are the first mixed-valence (of class III) transition-metal Zintl phases.