The plastic behavior of a series of amorphous semi-aromatic polyamides (SAPA-A) was investigated in compression mode at temperatures ranging from -100 degreesC to their glass transition temperatures. Data analysis was mainly based on the inspection of the temperature dependence of yield stress, sigma (y), plastic flow stress, sigma (pf), and strain softening, SSA = sigma (y) - sigma (pf). The activation volumes, V-0, and the index of non-elastic behavior, I, were also determined, to account for the sensitivity of plastic deformation to strain rate and for the relative easiness of elastic and non-elastic processes, respectively. Some connections have been established between the nature of the molecular motions and the values of sigma (y), sigma (pf), SSA, V-0, and I, thanks to the knowledge of the relaxation behavior of these materials. The role played by the secondary relaxation motions (and especially by those presenting a cooperative character) was highlighted, in agreement with our conclusions in earlier reports on the subject. In addition, the molecular analysis proposed for describing the plastic deformation of the SAPA-A materials was shown to hold for other polymers also bearing phenylene rings in the main chain, namely some semi-aromatic polyamides of another series (SAPA-R) and aromatic polycarbonates. Interestingly, the line of reasoning proposed here proved to be suitable also to account for the plastic deformation characteristics of various vinyl polymers and copolymers.