We determined by X-ray diffraction the structure of a slightly K-doped BaBiO3 single-crystal. The best refinements were obtained by describing the structure in the monoclinic I2/m space group similarly to the structure of the undoped compound. In this description the Bi cations occupy two crystallographically independent sites. We found that at this low K concentration, the K atoms are not, as expected, substituting for the Ba cations but are located on one of the two Bi sites. The two octahedral Bi sites are alternatively compressed and elongated along the c-axis. The formal valences of the two Bi cations deduced from the Brown and Altermatt formula are 3.79 and 4.44, respectively. This indicates that these cations are disproportionated into Bi3+ and Bi5+; however, they are only partially ordered.