Unresolved, spectrum or spectroscopic binaries containing F or G giant or supergiant primaries are being observed with the IUE satellite. In most cases the secondary is a B or early A star probably still on the main sequence. The temperature class of the secondary is determined accurately by UV spectral classification and/or analysis of the composite flux distribution. The flux analysis uses a large grid of intrinsic colors and yields a best trial-and-error fit for the spectral types of primary and secondary, their V magnitudes, and the interstellar reddening. From the temperature class of the secondary, its probable main-sequence luminosity leads to a luminosity estimate for the primary with an uncertainty of about one magnitude. Several single-lined spectroscopic binaries are also being observed at high dispersion (in collaboration with Ake, Fekel, Harvel, and Kondo), to get the differential velocity between components from which mass ratios can be determined and masses estimated for the supergiants.