Amorphous poly(diethylsiloxane) (PDES) elastomers undergo a transition to an aligned mesomorphic state when subjected to uniaxial tension. The structural changes associated with this transition and the kinetics of its formation have been investigated by in-situ synchrotron wide-angle and small-angle X-ray scattering. In the mesomorphic state, the PDES elastomers are biphasic, consisting of aligned mesophase domains and amorphous material. Because of the well-defined structure of the networks used, we were able to determine that the mesophase domain size is governed by the precursor chain length and is unaffected by trapped entanglements. The observed increase in mesophase content with increase in extension ratio in a fully necked sample is caused by an increase in the number of mesophase domains rather than an increase in domain size. In the extended state, PDES elastomers attain a very high degree of segment orientation comparable to that of mesogen-containing liquid crystalline elastomers.