Free-volume hole sizes and fractions in a series of thermosetting plastics (tetraglycidyl-diaminodiphenylmethane (TGDDM), diglycidyl ether of Bisphenol A (DGEBA) cross-cured with diaminodiphenyl sulfones (DDS)) are measured by using positron annihilation lifetime (PAL) spectroscopy. Relevant mechanical properties are measured in these polymers which have different configurational isomers and chain extensions of curing oligomers. Correlations between the obtained free-volume hole spectroscopy to determine materials parameters and engineering problems are discussed.