Use　of　tight　ultrafiltration　(UF)　membranes　has　created　a　new　pathway　in　fractionation　of　dye/salt　mixtures　from　textile　wastewater　for　sustainable　resource　recovery.　Unexpectedly,　a　consistently　high　rejection　for　the　dyes　with　smaller　sizes　related　to　the　pore　sizes　of　tight　UF　membranes　is　yielded.　The　potential　mechanism　involved　in　this　puzzle　remains　unclear.　In　this　study,　seven　tailored　UF　membranes　with　molecular　weight　cut-offs　(MWCOs)　from　6050　to　17530　Da　were　applied　to　separate　dye/salt　mixtures.　These　UF　membranes　allowed　a　complete　transfer　for　NaCl　and　Na2SO4,　due　to　large　pore　sizes.　Additionally,　these　UF　membranes　had　acceptably　high　rejections　for　direct　and　reactive　dyes,　due　to　the　aggregation　of　dyes　as　clusters　for　enhanced　sizes　and　low　diffusivity.　Specifically,　the　membrane　with　an　MWCO　of　7310　Da　showed　a　complete　rejection　for　reactive　blue　2　and　direct　dyes.　An　integrated　UF-diafiltration　process　was　subsequently　designed　for　fractionation　of　reactive　blue　2/Na2SO4　mixture,　achieving　99.84%　desalination　efficiency　and　97.47%　dye　recovery.　Furthermore,　reactive　blue　2　can　be　concentrated　from　2.01　to　31.80　g.L-1.　These　results　indicate　that　UF　membranes　even　with　porous　structures　are　promising　for　effective　fractionation　of　dyes　and　salts　in　sustainable　textile　wastewater　treatment.