University Department of Chemistry, T. M. Bhagalpur University, Bhagalpur-8 12 007, India Manuscript received 11 August 1998, revised 15 February 1999, accepted 17 May 1999 In view of the importance of concentration fluctuation in the long wavelength limit in the study of alloying behaviour of molten alloys, an attempt has been made to obtain expressions for Scc(o) from two different theories, i.e. quasi-lattice theory and electronic theory based on hard sphere model. The expression for Sm(o) from hard sphere model involves additive hard sphere diameter and non-additive potential depth. The above expression for Scc(o) has been utilised to investigate the alloying behaviour of Na—K molten alloys. Both the theories suggest occurrence of like atoms pairing (self-coordination), leading to segregation in Na—K molten alloys. The role of non-additivity of potential depth on Scc(o) has also been investigated with reference to Na—K system.