The problem came up in the cooling unit of veteran astronaut Rick Mastracchio's spacesuit after he re-entered the space station airlock following a spacewalk that lasted 5.5 hours, NASA said.
It was not believed to be the same type of issue that caused a dangerous water leak in the helmet of Italian astronaut Luca Parmitano in July.
An investigation into that situation is ongoing.
Faced with unexpected repairs due to an equipment breakdown at the orbiting lab on December 11, NASA arranged makeshift snorkels inside the 35-year-old spacesuits and absorbent pads in the helmets for these spacewalks in case such a leak happened again.
More From This Section
"This issue is not related to the spacesuit water leak that was seen during a July spacewalk."
A spokesman confirmed to AFP that the sublimator is the space suit's cooling unit.
Rather than allow Mastracchio, 53, to wear that suit again, astronauts are planning to work on resizing a spare spacesuit aboard the ISS for him on his next spacewalk to complete the ammonia pump module replacement.
NASA released the news late yesterday, after the spacewalk by the two American astronauts went faster than planned and appeared to go off without a hitch.
No live footage was broadcast, however, of the time astronauts spent inside the airlock after re-entering the space station following the spacewalk.
Astronauts made fast work of their key task for the day, disconnecting the old pump. They were also able to take on the extra task of removing the pump, which had been scheduled for tomorrow.
"Both Mastracchio and (Mike) Hopkins reported dry conditions repeatedly throughout Saturday's activities and the two were never in danger," NASA said in a statement.