The dielectric functions of NbxMo1−x alloys (x=0.2,0.5,0.8) and of Nb with 10-at.% Zr, with 20-at.% V, and with 20-at.% Ta were determined in the 0.1-25 eV energy range. Some of the interband region below 3 eV can be interpreted on the basis of the rigid-band model for Nb-Mo while the large structure at 4-4.5 eV cannot be so interpreted in any of the alloys using existing bands. An examination of all the alloys shows that there probably are distortions of the bands due to strain and potential differences. The transitions beginning at about 9 eV, from the Fermi level to a flat band above, are seen to have delocalized final states. All the alloys show two volume and two surface plasmons like those of Nb and Mo.