The large deviation from the ideal mixture behaviour and the concentration-dependent asymmetry in the thermodynamic properties of Mg-Sn liquid alloys is investigated within a simple theoretical model of heterocoordination (i.e. a model in which there is a preference for unlike atoms to be paired as nearest neighbours). This has been utilized to extract additional microscopic information such as concentration fluctuations and the chemical short-range order parameter. The analysis suggests that heterocoordination leading to the formation of chemical complexes Mg₂Sn is likely to exist in the melt but is only of a weakly interacting nature. The interaction energies between the species of the melt are found to depend considerably on temperature and the alloy is chemically more ordered towards the Mg-rich end of the phase diagram.