Carey, who collaborated with Houston, 48, on "When You Believe" – a song from the Prince of Egypt soundtrack – is "heartbroken and in tears" over Houston's passing, she wrote on Twitter Saturday.
Calling the former Bodyguard star "incomparable," Carey Tweeted her "heartfelt condolences to Whitney's family and all her millions of fans around the world."
Adds the singer, "She will never be forgotten as one of the greatest voices to ever grace the earth."
Other people close to Houston have also expressed their upset on the social networking site.
Hip-hop mogul Russell Simmons wrote that he is "deeply saddened" by her death, adding that she was "one of the greatest singers this world has ever heard."
Calling Houston a "legend," Christina Aguilera Tweeted: "love and prayers to Whitney's family ... She will be missed."
Tearfully reminiscing over the "first CD I ever bought," LeAnn Rimes recalls special interactions she had with Houston.
"I can't stop crying," she Tweeted. "I have a show to do soon. Wow, I learned how to do what I do because of 3 great women, one being Whitney. She was always so kind to me. This is not gonna be an easy show for sure. I'm doing what she taught me to do by listening to her records over & over as a little girl. Her death is very surreal."