BEECH 90 CARGO PLT DECLARED AN EMER AND DIVERTED TO LAND AFTER FINDING THAT FUEL WAS STREAMING OVERBOARD OUT OF EACH MAIN WING TANK DUE TO THE FUEL CAP MISSING FROM TANK AND THE OTHER HANGING BY A PLASTIC STRAP.

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TOOK OFF FROM DEN AT XA15. AFTER LEVELING AT 11000 FT MSL; DID MY SYS CHKS AND NOTED THE FUEL GAUGES SHOWED 1300 LBS IN EACH WING (FULL). FUEL ON TKOF WAS 900 LBS IN EACH WING. CHKED FUEL SYS CIRCUIT BREAKERS; BUT THEY WERE NORMAL. CHKED LEVEL IN NACELLE TANKS AND THEY WERE FULL (AS EXPECTED). APPROX 10 MINS LATER; I ENTERED IMC. SINCE THERE WAS AN AIRMET FOR ICE AND IT WAS DARK; I TURNED ON THE WING ICE LIGHTS TO CHK FOR ICE. AT THAT POINT; I NOTED FUEL STREAMING FROM THE FUEL FILLER INLET IN THE L WING. I CHKED THE R WING AND ALSO NOTED FUEL COMING FROM THE FUEL INLET. THE FUEL CAP WAS DISPLACED BUT ATTACHED TO THE AIRFRAME BY ITS PLASTIC CONNECTING STRAP. UPON FURTHER SCRUTINY; I COULD TELL THE L WING FUEL CAP WAS MISSING. I CONTACTED ZDV; DECLARED AN EMER AND REQUESTED AN IMMEDIATE VECTOR TO COS OR PUB; WHICHEVER WAS CLOSER. THE CTLR SAID THEY WERE EQUIDISTANT; BUT I REMEMBERED THAT PUB WAS NOTAMED CLOSED. AFTER CHKING; THE CTLR CONFIRMED THE CLOSURE; SO I WENT DIRECT TO COS. THE WX THERE WAS APPROX 3000 FT; 10 MI. I ELECTED TO LAND ON RWY 35R BECAUSE IT WAS CLOSEST. THE LNDG WAS UNEVENTFUL. FUEL HAD STOPPED SIPHONING FROM THE WINGS BEFORE LNDG; AND THE QUANTITY AFTER LNDG WAS 450 LBS IN THE L WING AND 750 LBS IN THE R. I HAD ORDERED FUEL AT DIA AFTER LNDG THE PREVIOUS NIGHT. THE FUELER HAD LEFT A TICKET FOR 80 GALLONS; BUT I WAS NOT PRESENT DURING THE REFUELING. I LOOKED AT THE FUEL CAPS DURING PREFLT BUT FAILED TO NOTICE THEY WERE NOT PROPERLY SECURED. I HAD REFUELED AT DIA MANY TIMES AND THE FBO THAT DOES THE REFUELING SEEMED TO HAVE VERY WELL TRAINED PEOPLE. MY PRIOR EXPERIENCE WITH THIS AND OTHER REPUTABLE FBO'S LULLED ME INTO TRUSTING THEM MORE THAN I SHOULD HAVE. GIVEN THE LIGHTING CONDITIONS ON THE CARGO RAMP AFTER DARK; I SHOULD HAVE MADE A MORE THOROUGH PREFLT CHK.