The structural properties and the Cd-distribution of CdSe layers with a nominal thickness between 0.5 and 3.6 monolayers (ML) in a ZnSe matrix were investigated by plan-view and cross-section transmission electron microscopy. All layers contain a high density of Cd-rich islands with sizes below 10 nm, which are completely embedded in a 3 nm thick CdZnSe wetting layer. Large islands with a density two orders of magnitude below the small island density are additionally observed at t(n) greater than or equal to 3 ML above the critical wetting layer thickness for the transition from the two-dimensional to the three-dimensional growth mode. Composition analyses on a subnanometer scale reveal the Cd-distribution and Cd-content, which increases in the wetting layer and in the small islands with the nominal layer thickness.