Using an electron diffraction technique, the thicknesses of the (nonorthorhombic) twin boundary layers were measured for pure and alloyed YBa2(Cu1−xMx)3O7−&dgr;. Boundary thickness varied from ∼0.7 nm forM=Ni,x=0.02, and &dgr;&bartil;0 to ∼2.6 nm forM=Al,x=0.02, and &dgr;&bartil;0, while it was ∼1.0 nm for a pure YBa2Cu3O7. High‐resolution transmission electron imaging of similar specimens supports the existence of such twin boundary layers.