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 12/m space group similarly to the structure of the undoped compound. In this description the Bi cations occupy two crystallographically independent sites occupied by disproportionated Bi3+ and Bi5+ cations, respectively. These cations are partially ordered. Surprisingly we found that for this low K concentration, the K atoms are located on one of the two Bi sites and are not, as expected, substituting for the Ba cations.