The body centered cubic (a∼4.26 Å)γ-type solid solution (ssγ) phase (1−x)Bi2O3·xCaO (∼0.50<x<∼0.60 at ∼820°C) was investigated by electron diffraction. Instead of finding evidence for additional Bragg reflections at G±a*as previously assumed attempting to interpret powder neutron diffraction data (Gruberet al., J. Solid State Chem.108, 250–252, 1994), localized diffuse scattering was detected and described by G±q, where q≈0.2〈211〉*γis a (diffuse) modulation wave vector. A crystal structural relationship between the cation arrays in the ssγphase and the low-temperature line phase, Ca4Bi6O13(LT), was established. The unit cell of the LT phase can be expressed as aLT=aγ−cγ, bLT=53(aγ−2bγ+cγ) and cLT=12(aγ+bγ+cγ). From the cation ordering in the LT phase and the reciprocal space relation 2b*LT=0.2〈121〉*γ≈q, the localized diffuse scattering in the ssγphase was linked to cation ordering into layers perpendicular to 〈121〉γ.