Dynamic mechanical and dielectric properties of substituted polypentenamers with phosphonate side groups and their hydrogenated derivatives have been studied. Methyl esters, acids, and salts were investigated at two concentrations, 6.5 and 11.1 mole percent. In the unhydrogenated deriv­atives, one principal relaxation, labeled fl, is observed mechanically in the temperature range from -160 to 100°C. This {J relaxation arises from micro-Brownian segmental motion accompanying the glass transition. Its temperature is substantially affected by the substituent concentration while its breadth is affected by the chemical nature of the substituent. An extended "rubbery plateau" region exists in the acid and salt derivatives. The dielectric results generally reinforce the mechanical assignments. In the hydrogenated derivatives, three relaxations labeled a, {J, and r in order of de­creasing temperature occur mechanically in this temperature range. The temperatures at which the a and fJ relaxations occur depend greatly on the chemical nature of the substituents, the sub­stituent concentration, and the thermal history of the sample; while the r relaxation appears to be independent of these variables. Suggested assignments for the relaxations observed in these poly­mers, based on the dual glass transition theory of Boyer for semicrystalline polymers, have been proposed. The dielectric results are consistent with the proposed assignments.