Dos Santos tipped the scale at 239 pounds, while Carwin came in at 254 for heavyweight number-one contender bout.
Meanwhile, Kenny Florian weighed in at 146 pounds for his featherweight debut against Diego Nunes, and features middleweight Mark Munoz and Demian Maia also made weight without problems.
Check out the complete UFC 131 weigh-in results below, and stop back tomorrow for live UFC 131 results starting at 7pm ET.
MAIN CARD (PPV)
- Junior dos Santos (239) vs. Shane Carwin (254)
- Kenny Florian (146) vs. Diego Nunes (145)
- Demian Maia (186) vs. Mark Munoz (186)
- John Olav Einemo (261) vs. Dave Herman (233)
- Donald Cerrone (155) vs. Vagner Rocha (155)
PRELIMINARY CARD (Spike TV)
- Sam Stout (155) vs. Yves Edwards (155)
- Jesse Bongfeldt (185) vs. Chris Weidman (186)
- Krzysztof Soszynski (205) vs. Mike Massenzio (201)
- Nick Ring (185) vs. James Head (186)
- Dustin Poirier (146) vs. Jason Young (145)
- Joey Beltran (247) vs. Aaron Rosa (261)
- Michihiro Omigawa (145) vs. Darren Elkins (145)
Pictured: Shane Carwin