The 18-year-old American, who had broken the 200 metres butterfly and 200 individual medley world marks in semi-finals earlier this week, came out with appetite undiminished and obliterated the 100 butterfly and his own 200 medley mark.
The 100 butterfly world record had stood for just a couple of minutes, with Andriy Serdinov of Ukraine clocking 51.76 seconds in the first semi-final only for Phelps to annihilate that time when he hurtled through the second semi-final in 51.47.
The previous mark of 51.81 belonging to Australia's Michael Klim had stood since December 1999.
Barely 50 minutes had elapsed before Phelps was back for the 200 individual medley final and destroying the 1:57.52 mark he had set the day before. He was way inside that record schedule all the way and beat it by more than a second in 1:56.04.
"After the 100 fly world record I was very, very fired up," said Phelps, who retained his 200 butterfly title on Wednesday and looks the best of bets to make it four individual golds with the 100 butterfly on Saturday and 400 individual medley on Sunday.
"I wanted to break it (the 200 medley record) again, so I just went out there and laid it on the line in the first 100 and tried to hold on. The first world record fired me up big time. I was pretty fired up, pretty pumped for that race."
THORPE MILESTONE
Ian Thorpe, for once not the centre of attention, came through for the silver in 1:59.66, thus equalling the biggest aggregate medal haul in world championship history, having joined Germany's Michael Gross on 13.
The Australian has 11 golds, one silver and one bronze, comprising two golds in 1998, a record six golds in 2001 and three golds, a silver and a bronze in the Palau Sant Jordi pool this week.
Olympic and defending world champion Massimiliano Rosolino of Italy took the bronze.
American Amanda Beard equalled the women's 200 metres breaststroke world record, coming through on the final length of the final to overhaul Australia's Leisel Jones, who had taken the field out at a breakneck pace.
Beard, double Olympic silver medallist in 1996, swept past her on the last length to touch in 2:22.99 to equal the world mark set by China's Qi Hui in Huangzhou in April 2001.
"I didn't really expect that at all. I kind of knew that world record was going to be around for a while, so it's amazing to be on the same label with that world record," said Beard.
Jones took the 200 silver while Qi took the bronze.
Olympic 1500 freestyle champion Grant Hackett, leading from start to finish, won the men's 800 freestyle title in the absence of 2001 champion Thorpe, who preferred to try the 200 individual medley.
The 23-year-old Australian, silver medallist last time, won in 7:43.82 from Larsen Jensen of the U.S. (7:48.09) and Ukraine's Igor Chervinskiy (7:53.15).
American Aaron Peirsol, winner of the 100 backstroke on Tuesday, picked up his second gold when he retained his 200 metres backstroke title with a commanding victory.
Former European junior champion Hanna-Maria Seppala became the first Finnish woman to win a world title when she won the 100 freestyle from Australia's Commonwealth champion Jodie Henry and American Jenny Thompson, who collected her 12th world championship medal with a bronze.
