The reaction between solid steel and liquid Al-Zn-Si alloy leads to the formation of a solid intermetallic interfacial layer. In the case of industrial Al-43.5Zn-1.5Si coated steel strips, the thickness of the intermetallic layer is on average 1.35 μm and the kinetics of the reaction is controlled by the effect of Si on Al and Fe diffusivities through the solid intermetallic layer. In this paper it is shown that the thickness of the intermetallic interfacial layer decreases as the Si content in the liquid alloy increases. EDXS microanalysis at the interface of industrial coated steel strips shows that the interfacial intermetallic compounds are chemically similar to those formed in the bulk of Al-43.5Zn-1.5Si liquid baths in continuous coating lines. Differential acid dissolution of the coatings reveals that the intermetallic layer is not planar at the interface with the coating overlay and is formed by grains of different size and chemical composition. Addition of minute quantities of Ti to the Al-Zn-Si liquid alloy causes changes in the morphology of the intermetallic layer and an overall refining of the microstructure.