Dielectric permittivity and loss have been measured over the frequency range 10(-2) Hz-10 kHz between 100 K and 350 K for samples of two mesomorphic side-chain poly(vinyl ether)s, poly(4-(11-(vinyloxy)undecyloxy)-4-ethoxyphenylbenzoate) (P-EtO) and poly(4-(11-(vinyloxy)undecyloxy)-4-cyanophenylbenzoate) (P-CN) of different degree of mesogen group orientation. X-ray scattering showed that P-EtO was in a semicrystalline state below 345 K, whereas P-CN displayed a tilted smectic structure at temperatures lower than 315 K. Four relaxation transitions were found in P-EtO : alpha, the glass-rubber transition occurring at 290-300 K, and three subglass processes referred to as beta, gamma and delta. P-CN exhibited only three dielectric processes, alpha, beta and gamma. The low temperature process, delta, was absent in P-CN and could be assigned to torsion about the pendent phenyl-carbon-ether-oxygen bond. Subglass processes beta and gamma exhibited an Arrhenius temperature dependence with activation energies of 60-105 and 32 +/- 2 kJ mol-1, respectively. The activation energy of the gamma process was insensitive to morphology and it was assigned to local motions in the spacer group. The activation energy of the beta process varied considerably among the studied polymers with higher values for the highly ordered P-EtO than for P-CN. It is suggested that the beta process leads to reorientation of the carboxylic group in the phenyl benzoate moiety.