The mechanical properties of transparent hybrid organic-inorganic nanocomposites made from siloxane and zirconium oxopolymers are investigated at two different length scales. The complex interface that associates the two phases is made of covalent Zr-O-Si bonds and hydrogen bonding. The rubbery properties studied by creep and recovery present specific behaviors in comparison with model elastomers. This is a result of the complex crosslinking units. The stress relaxation phenomenon has been studied at the molecular scale by H-2 quadrupolar NMR. During stress relaxation, the anisotropy of the molecular motion decreases slowly. This study demonstrates the straightforward relationship existing between the macroscopic and microscopic relaxation phenomena. (C) 2001 John Wiley & Sons, Inc.