Strain-rate sensitivity index, m, values of several thermoplastics (HDPE, PP, PMMA, PS, PVC, PC, and PA) were determined at ambient temperature by both variable strain-rate and stressrelaxation methods. Specimens were loaded in tension in the elastic portion of the stress-strain curve at various strain rates and the load was recorded as a function of elongation. Index values were determined from the relation \(m = [\partial \ln (\sigma )]/[\partial \ln (\dot e)]_{e, T} \). Specimens were also loaded in tension at constant strain rate to the proportional limit, loading was halted, and the load was recorded as a function of time at constant strain. A numerical algorithm was implemented to minimize the root-mean-square difference between an empirical equation and the experimental data, i.e. $$\Phi (n, \tau ) = \left( {1/N\sum\limits_i {\{ P_o \exp [ - (t_i /\tau )^n ] - P(t_i )\} } ^2 } \right)^{1/2} $$