An analysis of clause union from the theory of Relation Grammar can be used to investigate adversative construction in languages,such as in Indonesian as has been done by Kana (1986). On the basis of that assumption , this paper tries to show the uniqueness of adversatives in Acehenese by employing a clausa union approach. The result of the investigation suggest that adversative like aneuk nyan ka ji-tamong jen ('that child is possessed by an evil spirrit') needs to be reconsidered so that it is in accordance with the theory. It is not case with another kind of adversative which obtained through the rise of possessor like aneuk nyan ka gadoh kense ('that child lost his pencil').The latter kind of adservative does not require any modification of the clause union analysis.