AN A319 FLT CREW EXPERIENCE A LOSS OF CABIN PRESSURE WHICH LEADS TO AN EMER LNDG AT LAX.

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UPON LEVELING OFF AT FL390 THE FO POINTED TO THE ECAM PRESSURE PAGE. WE NOTICED THAT THE CABIN WAS CLBING SLOWLY THROUGH 9000 FT. CABIN REACHED 10000 FT. ALARM WENT OFF. DONNED OXYGEN MASKS. STARTED EMER DSCNT AFTER COORD WITH ATC. CHKLIST WAS ACCOMPLISHED. NOTICED AT THE BEGINNING OF THE DSCNT THAT THE CABIN STARTED TO STABILIZE. LEVELED AT FL270 INITIALLY. PRESSURE SYS APPEARED TO BE WORKING NORMALLY. ASSESSED SIT WITH PURSER. SHE INDICATED EVERYTHING WAS FINE AND NO MASKS HAD DROPPED; PAX WERE OK AND FLT ATTENDANTS WERE OK. DECIDED TO RETURN TO LAX. DECLARED EMER. INFORMED DISPATCH. MADE PA ANNOUNCEMENT TO PAX INDICATING THE NATURE OF THE PROB AND THE FACT THAT WE WERE RETURNING TO LAX. CALLED PURSER AGAIN AND ASKED ABOUT THE CONDITION OF THE PAX AND FLT ATTENDANTS AND INDICATED TIME TO LNDG 17 MINS. SHE SAID ALL WAS NORMAL IN THE CABIN. MADE AN UNEVENTFUL APCH AND LNDG TO RWY 25L AT LAX. SECURED THE EMER EQUIP. SUPPLEMENTAL INFO FROM ACN 596396: DURING THE DSCNT CABIN PRESSURE BEGAN TO DSND. BY FL270 CABIN PRESSURE RETURNED TO NORMAL INDICATING APPROX 6500 FT.