LJ35 CAPT RPTS SMOKE IN COCKPIT DURING DSCNT FOR LNDG. OXYGEN MASKS ARE DONNED AND AN EMER IS DECLARED. SITUATION STABILIZES AS THE FLT CREW LANDS AT DEST ARPT.

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WE WERE CLRED FROM FL410 TO FL230. DURING THE DSCNT FROM CRUISE ALT; SMOKE BEGAN TO COME OUT OF THE OVERHEAD COCKPIT VENTS. A FEW SECONDS LATER; MORE SMOKE WAS POURING IN THE COCKPIT. WE PUT ON OUR OXYGEN MASKS AND LOOKED BACK TOWARDS THE CABIN. THERE WAS NO SMOKE THERE. THE SMELL WAS OF OVERHEATED ELECTRICAL/INSULATION. WE DECLARED AN EMER AND REQUESTED FURTHER DSCNT TO ZZZ WHICH WAS THE CLOSEST ARPT; ABOUT 35 NM AHEAD. WE WERE GIVEN A LOWER ALT (14000 FT) AND A HDG. LATER WE WERE GIVEN A FEW MORE STEPDOWNS; ANOTHER HDG; INSTRUCTIONS TO JOIN THE ILS AT 5700 FT AND CLRED TO LAND. WE REQUESTED THAT THE FIRE TRUCKS STAND BY. THERE WAS NOW SMOKE IN THE CABIN AS WELL BUT IT DIDN'T LOOK LIKE IT WAS GETTING THICKER. WE CANCELED THE EMER ON SHORT FINAL; LANDED AND TAXIED OVER TO THE FBO WHERE THE FIRE TRUCKS MET US. I SHUT DOWN THE ENGS AND TURNED OFF THE BATTERIES WHILE THE COPLT DEPLANED OUR PAX. THE SMOKE HAD STOPPED. THE FIREMEN INVESTIGATED THE CABIN AND FOUND THE AREA AROUND THE CABIN FAN TO BE VERY HOT. THE ACFT BATTERIES WERE DISCONNECTED; THEY VENTILATED THE CABIN AND STOOD BY FOR A WHILE. WE UNLOADED THE BAGS AND SENT OUR PAX ON THEIR WAY.