A discrete spectra transformation technique is used for the processing and analysis of long-term stress relaxation and creep compliance data of mineral-filled polymer composites. A non-linear regression simultaneously adjusts the parameters of N discrete relaxation or retardation spectra. For small N the solution is insensitive to the choice of regression starting value sets. From the relaxation time spectrum a corresponding discrete retardation spectrum and creep compliance can be calculated using the Laplace transform and vice versa. The analysis of long-term (more than 1200 days) both relaxation and retardation experimental data demonstrates the applicability of the transformation technique. Comparisons of the experimental and calculated spectra are given. The influence of the filler amount is demonstrated.