Having lost top spot to Deportivo last week, Valencia were in no mood to relinquish their title aspirations, however, they were rocked by De Boer's sixth minute strike.
The Dutch international strode majestically through a strangely empty midfield before firing a 35 yard effort that swerved viciously and wrong-footed Santiago Canizares who dived in vein after the ball had rocketed into the net.
After such an early goal the game descended into a vicious, niggly affair and it was not a surprise that cards were being shown at regular intervals.
However, Gerard should have doubled the visitors' lead in the 15th minute when he found himself in space, but could only head Josep Guardiola's corner wide.
Valencia went up the other end and could have equalised as a brilliant turn by John Carew gave him space in the box but he flashed his shot wide.
Barca's five man midfield frustrated the home side, but this was no excuse for Fabian Ayala's pre-meditated stamp on young Portuguese star Simao which the referee adjudged to have been an accident.
Replays showed this was anything but the case and having already been booked Ayala was indeed lucky to stay on the pitch.
Having been booked for a clash with Gaizka Mendieta midway through the first-half, Luis Enrique was sent off on the stroke of half-time for a late tackle.
Hector Cuper's team-talk must have revolved around taking advantage of the extra man and avoiding doing something stupid to allow the referee to even the sides up.
Sadly for Cuper Amadeo Carboni paid little heed to his instructions and got himself sent off, also for two yellow cards for a clumsy challenge.
Canizares redeemed himself with two excellent saves from Patrick Kluivert, denying the Dutchman on a one-on-one chance and with a low shot that the keeper beat out.
In the closing stages Carew and Mendieta went close but a mixture of blocks and poor finishing kept the home side off the scoresheet.
Barca had not been beaten in 13 games but with such an attacking minded style it was a shock that they held out for a cleansheet.
With the result, Barca move into second on 36 points whilst Valencia drop behind the Cataluynians to third.
