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In this work we present a theoretical study of the effect of disorder on spin polarisation at the Fermi level, and the disorder formation energies for CoFeMnSi (CFMS) alloys. The electronic calculations are based on density functional theory with a Hubbard U term. Chemical disorders studied consist of swapping Co with Fe/Mn and Co with Si; in all cases we found these are detrimental for spin polarisation, i.e., the spin polarisation not only decreases in magnitude, but also can change sign depending on the particular disorder. Formation energy calculation shows that Co-Si disorder has higher energies of formation in CFMS compared to CoMnSi and CoFeSi, with maximum values occurring for x in the range 0.5-0.75. Cross-sectional structural studies of reference CoMnSi, CoFeMnSi, and CoFeSi by Z-contrast scanning transmission electron microscopy are in qualitative agreement with total energy calculations of the disordered structures.