We　studied　the　isothermal　adsorption　of　naphthalene　on　a　viscose-based　activated　carbon　fiber　(VACF)　in　aqueous　solution.　The　adsorption　experimental　data　were　fitted　to　adsorption　equations　such　as　the　Langmuir,　Freundlich,　Dubinin-Radushkevitch(D-R),　Dubinin-Astalov(D-A),　Langmuir-Freundlich(L-F),　and　Redlich-Peterson　(R-P)　equations.　The　effects　of　temperature,　pH,　and　pre-loaded　copper　ions　on　the　VACF　on　the　adsorption　of　naphthalene　were　investigated.　The　fitting　results　show　that　the　micropore　volume　filling　theory　fits　better　than　the　single-layer　adsorption　theory　for　naphthalene　adsorption　onto　VACF.　The　adsorption　of　naphthalene　occurs　spontaneously　between　20　and　40　degrees　C.　The　adsorption　capacity　of　VACF　for　naphthalene　decreases　significantly　above　30　degrees　C.　A　new　mechanism　is　proposed　to　explain　why　the　adsorption　capacity　increases　significantly　in　lower　pH　aqueous　solutions.　The　reduction　of　naphthalene　takes　place　on　the　VACF　with　pre-loaded　copper　ions.