Canetti and Fischlin have recently proposed the security notion universal composability for commitment schemes and provided two examples. This new notion is very strong. It guarantees that security is maintained even when an unbounded number of copies of the scheme are running concurrently, also it guarantees non-malleability, resilience to selective decommitment, and security against adaptive adversaries. Both of their schemes uses (k) bits to commit to one bit and can be based on the existence of trapdoor commitments and non-malleable encryption. We present new universally composable commitment schemes based on the Paillier cryptosystem and the Okamoto-Uchiyama cryptosystem. The schemes are ecient: to commit to k bits, they use a constant number of modular exponentiations and communicates O(k) bits. Further more the scheme can be instantiated in either perfectly hiding or perfectly binding versions. These are the rst schemes to show that constant expansion factor, perfect hiding, and perfect binding can be obtained for universally composable commitments.