The microstructure of mixed phase, c-axis oriented Tl–Ca–Ba–Cu–O thin films, exhibiting excellent superconducting properties, has been examined by transmission electron microscopy and x-ray diffraction. Although the stoichiometry of the as-deposited films corresponds to the Tl2Ca2Ba2Cu3O10 phase (Tl-2223, c = 3.6 nm), x-ray diffraction showed these films contain up to 50% of the Tl2CaBa2Cu2O8 phase (Tl-2122, c ≍ 2.9 nm). The x-ray diffraction peaks of the 3.6 nm phase are noticeably broader than those of the 2.9 nm phase, suggesting a smaller coherent size along the c-axis for the 3.6 nm phase Lattice-fringe images of the 2.9 nm phase typically show only an occasional or no defect layer; however, such layer defects are more abundant in the 3.6 nm phase and, though random, occur with sufficiently wide separation and regularity to be consistent with an average coherency of ∼40 nm. The coherent sizes along the c-axis, as determined by x-ray line profile analysis, were 140 nm and 40 nm, respectively, for the 2.9 nm and 3.6 nm phases, in good agreement with those determined by electron microscopy. The estimated width of the layer defects observed in the lattice-fringe images is suggestive, for the most part, of the four Cu-layered phases (Tl-1324 or Tl-2324) and to a lesser degree the two Cu-layered phases (Tl-1122 or Tl-2122).