As　the　triple　point　of　water　is　of　great　importance　for　the　International　Temperature　Scale　of　1990　(ITS-90)　and　for　the　definition　of　the　unit　of　thermodynamic　temperature,　its　long-term　stability　has　attracted　a　great　deal　of　attention.　In　a　study　of　long-term　stability,　a　mystery　has　been　uncovered.　Some　triple-point-of-water　cells　remain　stable　for　many　decades,　while　others　decrease　with　increasing　age　of　the　cells,　which　is　called　long-term　drift.　To　investigate　this　mystery,　we　used　cells　with　different　manufacture　dates　ranging　from　1974　to　2002　and　compared　their　analyses,　which　were　done　in　1984　and　2003.　Using　the　same　model　of　long-term　drift　as　that　used　by　Hill,　the　long-term　drift　rates　of　the　two　data　sets　are　-4.7　mu　K　.　year(-1)　and　-9.2　mu　K　.　year(-1),　respectively.　One　is　consistent　with　the　observed　depression　of　about　-4　mu　K　.　year(-1)　measured　by　Hill,　whereas　the　other　differs　greatly　from　Hill＇s　result.　In　addition,　corresponding　factors　influencing　long-term　drift　are　discussed　in　this　paper.