We studied the interactions between casein micelles and (depolymerised) guar gum, degraded by heating its aqueous solutions at low pH. The guar samples were characterised by a combination of size-exclusion chromatography and light scattering, which yielded the distribution of molar mass and corresponding radius of gyration. The relation between molar mass and radius of gyration showed that guar can be described as a random coil. Mixing guar gum with casein micelles led to a phase separation. This phase separation originates from a depletion interaction leading to an effective attraction between the casein micelles by non-adsorbing guar. The polymer concentration at the phase boundary increases with decreasing guar chain length. Vrij's theory for depletion interaction was used to calculate the phase boundary from the size of the casein micelles and the size and molar mass of the guar samples. The theory of Vrij predicts the trends for the phase boundary with respect to the molar mass dependence as found experimentally.