The present study intends to elucidate the effect of treatment solution composition on the formation and structure of Trivalent Chromium-based Conversion (TCC) coatings containing cobalt on zinc substrates. Model solutions with two different complexing agents, viz. fluoride and oxalate, with and without cobalt were applied to zinc plated steel. The scanning electron microscopy and atomic force microscopy images revealed a morphology with microstructural defects that can be improved to a more uniform and adherent structure by adding cobalt to the passivating bath. The elemental composition of the layer was investigated by auger electron spectroscopy (AES). Furthermore, the amounts of Cr and Co in the coatings were also measured with the aid of inductively coupled plasma optical emission spectroscopy (ICP-OES). In good agreement with AES, cobalt was also detected in the layers via ICP-OES measurement. The results of accelerated corrosion tests suggested that the formation of a densely packed layer is crucial for a good corrosion resistance of the coating.