A B757 INFLT DEVELOPS AN ODOR IN THE CABIN OVER THE WING AREA THAT SMELLS SIMILAR TO GAS. CABIN AND FLC INVESTIGATE AND ARE UNABLE TO IDENT ODOR; SO CAPT ASSUMES IT IS A GASOLINE SPILL AND DECLARES AN EMER TO GET ACFT ON GND; AS SOON AS POSSIBLE.

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LOCATION: ORD/190/200. 2 HRS 50 MINS INTO THE FLT A FLT ATTENDANT INFORMED US THERE WAS A STRONG SMELL OF GASOLINE IN THE CABIN AROUND ROW XX. I SENT THE FO BACK TO INVESTIGATE AND HE RETURNED STATING THAT THERE WAS A SMELL OF SOME KIND BUT HE COULDN'T TELL IF IT WAS FUEL; HE THOUGHT IT SMELLED MORE LIKE PAINT THINNER. WE DECLARED EMER AND HEADED STRAIGHT FOR ORD. THERE WAS NO INDICATION OF ANY PROB IN THE COCKPIT. WE CYCLED PACKS OFF ONE AT A TIME WITH NO REAL RESULT. APCH AND LNDG WAS UNEVENTFUL. CFR WAS STANDING BY AND I HAD THEM FOLLOW US TO THE GATE AND STAND BY UNTIL ALL THE PAX HAD DEPLANED. ONE OF THE PAX DEPLANING STATED THAT HE THOUGHT IT SMELLED A LOT LIKE INSECTICIDE. I INFORMED MAINT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE ODOR WAS RPTED BY A PAX. A FLT ATTENDANT DESCRIBED IT AS A GAS ODOR; BECAUSE IT SMELLED LIKE HER LAWN MOWER. ANOTHER PAX THOUGHT IT SMELLED LIKE AN INSECTICIDE. THE CAPT HAD THE CABIN CREW AND HIS FO CHK THE SEAT AREA AND OVERHEAD BINS TO FIND IF ANY LEAKING CONTAINERS OR DAMP BAGGAGE HAD RESULTED FROM A LEAKING CONTAINER OF FLUID. THE ACFT WAS FLYING FROM MEXICO BUT NO INSECTICIDE SPRAYING HAD OCCURRED; SINCE THE REQUIREMENT FOR SPRAYING IN MEXICO HAS BEEN DISCONTINUED. CAPT WAS UNABLE TO FIND ANY FINAL RESOLUTION ABOUT WHAT CAUSED THE ODOR; HE DID REQUEST INFO FROM HIS COMPANY TO FOLLOW UP; BUT; HE HAS RECEIVED NO FURTHER INFO. UPON LNDG; HE WAS ESCORTED BY THE CFR TO HIS GATE. HE WAS UNABLE TO GET OFF HIS ACFT TO INVESTIGATE BELOW THE CABIN FLOOR AS HE HAD TO PASS THROUGH CUSTOMS. IT IS THOUGHT BY FLC MEMBERS THAT THE ODOR RESEMBLED GASOLINE; PAINT THINNER; OR INSECTICIDE.