Ultrahigh temperature (UHT) processed and lactose-free milk are for export to Eastern markets. Free galactose and glucose in lactose-free milk increase occurrence of Maillard products and hence lower milk quality. Notably, enzymatic conversion of galactose into galacto-oligosaccharides (GOS) mitigates such undesirable reactions. As little is known about the effect of GOS on caseins, interactions between caseins in presence or absence of GOS or lactose were assessed by surface plasmon resonance analysis. Based on steady state binding constants (KD) soluble aS-casein showed one order of magnitude lower affinity than soluble b-casein and k-casein for chip-immobilised aS-and β-casein. By contrast, aS-casein had higher affinity for immobilised k-casein, followed by b- and k-casein. The interaction between the caseins did not obey a 1:1 binding model and it was not possible to calculate a precise stoichiometry. Importantly, lactose and GOS exerted very modest or no effect on the various interaction between the caseins.