Scanning transmission electron microscopy (STEM) (Z-contrast imaging and convergent beam electron diffraction, CBED) is applied to characterise III/V epitaxial heterostructures consisting of InGaAs-, InAlAs- and InP-compound layers grown by metal-organic vapour phase epitaxy (MOVPE) in a non-gaseous source configuration. The results concerning layer-widths, compositional changes and build-in strains are compared to those obtained by high resolution-X-ray diffraction. When the electron beam is positioned at an interface or at ultra-fine layers, the convergent beam electron diffraction pattern become depended on the conditions of the focussing electron lens(es). These conditions are in particular the focus-setting and the spherical aberration. Both can be measured by fitting a "simple" geometrical pattern to the experimental one. By taking these consideration into account a CBED-strain-measurement is shown with a high spatial resolution of 0.8 nm.