The space station uses chilled liquid ammonia to cool down the power systems on its eight giant solararray panels. A minor leak of this ammonia was first noticed in 2007, and NASA has been studying the issue ever since. In November 2012 two astronauts took a spacewalk to fix the problem, rewiring some coolant lines and installing a spare radiator due to fears the original radiator was damaged by a micrometeorite impact.

via Space Station Leaking Vital Coolant, NASA Says | Weather Underground.

Update: Astronauts Complete Spacewalk to Repair Ammonia Leak

A little more than 2 1/2 hours into the spacewalk, Cassidy and Marshburn removed the 260-pound pump controller box from the P6 truss and replaced it with a spare that had been stowed nearby on the port-side truss, or backbone of the station. Mission Control ran the new pump while the spacewalkers watched for any ammonia snowflakes, but so far there have been no new signs of a leak. Long-term monitoring of the pump will be required to determine whether the pump replacement has fixed the leak.