The dielectric alpha-relaxation of a series of linear polymethylphenylsiloxanes (PMPSs) has been investigated as a function of frequency in the temperature range 155.5-262 K. The breadth of the relaxation process, according to the empirical Kohlrausch-Williams-Watts and Havriliak-Negami functions, increases as the molecular weight increases. The apparent activation energy passes through a maximum at n(n), = IO which reflects the contribution of the whole molecule rotation to dipolar relaxation. A maximum in the dipole moment ratio occurs at n(n) similar to 50 due to dipole reinforcement. A similar investigation of a cyclic PMPS with n(n) = 10 indicates the effectiveness of dipolar cancellation in small siloxane rings.