A DC9-10 WAS RPTED TO HAVE FUEL VENTING FROM THE R WINGTIP. FUEL IN L TANK 9100 LBS AND R WING 8300 LBS. CTR TANK PUMPS OFF VENTING STOPPED.

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WE WERE ACR X TO ZZZZ. NORMAL PREFLT; START; TAXI. AFTER TKOF; WE TURNED N ON VECTORS TO THE SID. AS WE PASSED 3500 FT MSL; LEAD FLT ATTENDANT INFORMED US OVER INTERPHONE THAT A PAX RPTED LIQUID (FUEL) VENTING FROM R WINGTIP. FUEL WAS 9200 LBS PER EACH WING AND 800 LBS IN CTR TANK AT TKOF. WHEN VENTING WAS RPTED; FUEL WAS 9100 LBS L WING; 0 LBS IN CTR; AND 8300 LBS IN R WING. CTR PUMPS WERE TURNED OFF AND VENTING RPTEDLY STOPPED AFTER THAT. LEAD FLT ATTENDANT TOLD US THAT THE PAX WAS A PLT AND HE FLEW THE B757 FOR A FOREIGN COMPANY. HE EVEN DREW US A PICTURE OF THE R WINGTIP THAT DESCRIBES THE FUEL AND VAPOR COMING FROM THE R WINGTIP AREA. ACFT WAS XFERRED TO THE FO AT THIS POINT. SINCE THE ACFT HAD A RECENT HISTORY OF FUEL PROBS AND THE PAX AND FLT ATTENDANT'S OBSERVATION WERE VERY CREDIBLE; I CALLED DISPATCH AND MAINT CTL WHILE THE FO WORKED ON GETTING US A CLRNC BACK TO ZZZ INSTEAD OF GOING ON TO ZZZZ. DISPATCH AND MAINT CONCURRED WITH DECISION TO RETURN. WE BALANCED THE MAIN FUEL TANK QUANTITIES; THEN RETURNED TO ZZZ FOR AN UNEVENTFUL LNDG. I SPOKE WITH THE OBSERVING PLT AFTER THE FLT REVERIFIED WHAT HE OBSERVED AND THANKED HIM FOR HIS ALERT ASSISTANCE.