Analyzed in this paper are the deformation relationships between temperature; stress, strain; and stress rate in polyethylene terephthalate (PET) near the glass transition temperature. The uniaxial stress rate of inelastic deformation was found to be proportional to the square of the effective stress. A physical model of two simultaneous rate-activated processes with a general activation energy barrier is utilized. We call this “second-order kinetics.” Experimental data obtained by measuring load rate versus load point displacement were analyzed to describe stress relaxation of thin uniaxial strips. The activation enthalpy and activation volume show that the rate-controlling process is second-order.