In this work, theoretical and experimental investigations of the rheological and microstructural behaviour of semi-solid AlCu10%wt of volume solid fraction 18-40% during isothermal shear 1-1000 1/s were performed. This was used to evaluate the connections between the microstructural and rheological behaviours. From the microstructural analysis of the investigated system, it was observed that at low shear rates, the primary solid forms a porous, percolated network. At high shear rates the solid forms separate, densely packed cellular clusters. Agglomeration was observed at all levels of shear, but in different manners depending on the rate of shear. The agglomeration process was found to be partly reversible by de-agglomeration at increasing shear, but it also contributed to irreversible coarsening by sintering and coalescence of the structure. It was discovered that the crystallographic misalignment in the agglomeration process decreases with increasing shear rate, so that at high shear rates the clusters become monocrystalline. The investigated SSM macroscopic flow behaviour was as expected found to be thixotropic (shear thinning in the steady-state). This was semi-quantitatively linked to de-agglomeration and agglomeration processes. In conclusion, the presented results give strong empirical indication of the relation between agglomeration and de-agglomeration of the microstructure and the macroscopically observed thixotropy of SSM.