The effect of various dipolar and non-polar diluents on the dielectric α relaxation of methylphenylsiloxane oligomer has been studied. All the diluents showed a plasticising action by shifting the loss peaks to higher frequencies or lower temperatures with increasing concentration with a gradually decreasing activation energy. The normalised plots of ε″/ε″m against log(ƒ/ƒm) broadened with increasing concentration of diluent but retained their asymmetry and conformity to the empirical decay function for charge ϕ(t)= exp[–(t/τ0)β], the plotes of β against molar concentration usually being approximately linear. Increasing the bulky nature of the low polarity diluents produced greater changes in the dielectric parameters, the greatest effect being observed with straight chain arrangements. The dipolar diluents produced much greater reduction in β than ones which were non-polar or of low polarity. The broadening of the loss curves was explained in terms of a distribution of local concentration of diluent. Calculated values for the range of concentration ΔC were larger for the dipolar diluents than for those of low polarity and generally increased as the size of low polarity molecules increased. The magnitude εs–ε∞ of the α relaxation was little affected by the non-polar or low polarity diluents, but the presence of dipolar diluents increased it appreciably. The values for ε∞ for the oligomer + dipolar diluent systems were also larger than those for the pure polymer, suggesting the presence of a separate higher frequency relaxation for the diluent molecules, independent of co-operative motion between polymer and diluent molecules.