The cationic distribution of the solid solution BaFe 4 − 2 x Sn 2 + x Co x O 11 (0≤x≤2) has been determined by using combined neutron and x-ray diffraction. A very strong cation site selectivity has been found: Sn atoms do not enter 4f(1/2) sites at all, but this site is occupied by both Fe and Co atoms. On the other hand Sn atoms exhibit a strong preference for the 4e sites where Co atoms do not enter in any case. 6g sites are occupied by the three cations without any preference. No long-range magnetic ordering has been detected in any of the samples, but short-range three-dimensional ferrimagnetic ordering occurs below a temperature T N that decreases as x increases. Below T N down to T F , the system is composed of a set of ferrimagnetic clusters of finite size. A second magnetic transition to a spin-glass-like phase is observed in all the samples at T F , and this transition is attributed to the intrinsic frustration of the 6g sites arranged in a kagome$aa— lattice.