90° ferroelectric domain boundaries were observed by direct transmission electron microscopy in single crystals of pure BaTiO3and (Ba,Pb)TiO3solid solutions. Domain‐wall thickness measurements were made from electron image plates by a densitometer‐trace‐curve‐fitting technique. Boundary widths varied from 50 to 150 Å. These results are compared with theoretical estimates of 90° domain‐wall thicknesses and correlated with the relative purity levels of the crystals. The temperature variation of the 90° wall thickness is reported for the (Ba,Pb)TiO3solid solutions.