Energy-loss spectra were measured by backscattering 500-eV electrons from films of W${\mathrm{O}}_{3}$ grown on single-crystal W(100), and from single crystals of ${\mathrm{Na}}_{x}$ W${\mathrm{O}}_{3}$ ($0.35<x<0.86$) and Re${\mathrm{O}}_{3}$ which were cleaved in ultrahigh vacuum. The spectrum of a crystal with $x=0.61$ was nearly identical to the surface loss function $\mathrm{Im}[\ensuremath{-}{(\ensuremath{\epsilon}+1)}^{\ensuremath{-}1}]$ calculated from $\ensuremath{\epsilon}$ measured by optical reflectivity on a crystal with $x=0.65$. For $x=0.61$, two plasmons are observed, ${\ensuremath{\omega}}_{\ensuremath{-}}=1.90$ eV, and ${\ensuremath{\omega}}_{+}=6.5$ eV. ${\ensuremath{\omega}}_{\ensuremath{-}}$ scales approximately as ${x}^{\frac{1}{2}}$ and is assigned to the conduction-electron resonance. The plasmon lifetime ${\ensuremath{\tau}}_{p}=1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ sec determined from the observed linewidth agrees well with a value for the conduction-electron relaxation time ${\ensuremath{\tau}}_{e}=2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ sec we obtained from previously reported reflectivity data, but is shorter than a value ${\ensuremath{\tau}}_{c}=6.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ sec we estimated from previously reported dc conductivity data. The higher-energy plasmon ${\ensuremath{\omega}}_{+}$ is assigned to a screened longitudinal resonance of an interband excitation near 5 eV. ${\ensuremath{\omega}}_{+}$ is essentially independent of $x$ for $0\ensuremath{\le}x<0.9$. By fitting $\ensuremath{\epsilon}$ between 0 and 10 eV with a model dielectric function, we found a value for the mean effective conduction electron mass ${m}^{*}=0.80 {m}_{e}$, the interband contribution to the static dielectric constant ${\ensuremath{\epsilon}}_{s}=4.35$, and values for parameters characterizing the interband resonance. Taking these latter parameters to be independent of $x$, we calculated the bulk and surface loss functions for different $x$ values. These calculations indicate that mode hybridization is unimportant for $x<1$ in the bronzes. The effects of damping or joint density-of-states width, screening, and mode interference on the plasmon spectra are discussed. It is shown that Re${\mathrm{O}}_{3}$ and Ag metal have dielectric and energy-loss functions which are similar to the tungsten bronzes. However, some of the assignments given in the literature as to the identities of the conduction-electron and interband plasmons are interchanged from those given above for the bronzes. We believe that the conduction-plasmon hybridization with interband resonances is less important in those materials than previously realized.