  We present a comprehensive study on the silver bismuthate Ag$_2$BiO$_3$,
synthesized under high-pressure high-temperature conditions, which has been the
subject of recent theoretical work on topologically complex electronic states.
We present X-ray photoelectron spectroscopy results showing two different
bismuth states, and X-ray absorption spectroscopy results on the oxygen
$K$-edge showing holes in the oxygen bands. These results support a bond
disproportionated state with holes on the oxygen atoms for Ag$_2$BiO$_3$. We
estimate a band gap of $\sim$1.25~eV for Ag$_2$BiO$_3$ from optical
conductivity measurements, which matches the band gap in density functional
calculations of the electronic band structure in the non-symmorphic space group
$Pnn2$, which supports two inequivalent Bi sites. In our band structure
calculations the disproportionated Ag$_2$BiO$_3$ is expected to host Weyl nodal
chains, one of which is located $\sim$0.5~eV below the Fermi level.
Furthermore, we highlight similarities between Ag$_2$BiO$_3$ and the well-known
disproportionated bismuthate BaBiO$_3$, including breathing phonon modes with
similar energy. In both compounds hybridization of Bi-$6s$ and O-$2p$ atomic
orbitals is important in shaping the band structure, but in contrast to the
Ba-$5p$ in BaBiO$_3$, the Ag-$4d$ bands in Ag$_2$BiO$_3$ extend up to the Fermi
level.
