The free volume in epoxy resins is strongly connected with the cross-linking process of resin and hardener. In-situ measurements of conventional positron annihilation lifetime spectroscopy (PALS) were carried out to investigate the free volume during post-curing of the epoxy resin system diglycidil ether of bisphenol A (DGEBA) and diethylenetriamine (DETA). Direct comparison with the results of infrared spectroscopy (IR) measurements in the transmission mode was performed. The decrease of the free volume was highly dependent on the composition of resin and hardener, since excess hardener leads to qualitative different curing mechanisms.