Molecular dynamics is used to analyze the motions of backbone and lateral groups that lead to the formation and dissociation of excimer-forming conformations in methylphenylsiloxane. Several structures are studied: two fragments of linear poly(methylphenylsiloxane) with 14 (P14) and six (P6) monomer units, the cyclic trimer (C3) in its two configurations, cis and trans, and the disiloxane (DS) as a model for a single diad. It was found that the distribution of distances between C-1 atoms in a dad of any structure and the corresponding rotational angle distribution do not show the clear distinctions between different states that are typical of RIS models. Nevertheless, excimer-forming sites can be defined as diad conformations with distances between neighbor C1 atoms below 4.2 Angstrom, which correspond to ample ranges of the rotational angles (0-110 degrees in DS), to angles between phenyl rings of about 15 degrees with a certain staggering of the aromatic groups, and to angles between Si-C-1 bonds in neighbor rings below 45 degrees. This conformation of the excimer-forming site is very different from the one observed in hydrocarbon analogous polymers. Large differences can also be observed in the set of movements necessary to form excimer sites in both types of polymers. In DS, P14, and P6, the two bonds of a dad rotate in a coordinated way, keeping the sum of dihedral angles about constant. Changes of phi(1) + phi(2) from any value to 30 degrees are accompanied by changes in the angle between phenyl rings from around 90 degrees to 15 degrees and by the approach of the two chromophores to a distance smaller than 4.2 Angstrom. During this time, phi(1) and phi(2) may suffer more than 20 random changes. These complex chain motions are quite different from the simple rotation through a backbone bond where either phi(1) or phi(2) changes by 120 degrees, which is the type of motion usually associated with the excimer formation in polystyrene-like chains. Transitions to form excimer sites are also discussed in term of time-dependent distributions of the conformational variables. The frequency of transitions between excimer sites and the other conformational states is larger for C3 and for DS than for P14 and P6, and calculated relative values of the rate constant for excimer formation are in agreement with photophysical experimental results. The distribution of transition times can also help to interpret the existence of a fraction of isolated chromophores in the polymer.