The microstructure and the flexural behaviour of two highly crosslinked epoxy mixtures have been investigated. Besides the usual alpha and beta relaxations appearing for any epoxy mixture, an additional omega relaxation, located slightly above room temperature, was evident for the mixture with higher crosslink density. This relaxation, which seems to be related to microstructural variations originating in a free volume increase, permits us to explain the lower mechanical properties of the larger higher crosslinked mixtures in the temperature range between the beta and alpha relaxations. Whilst at room temperature the flexural modulus did not show any significant variation upon testing rate and the strength slightly increased for both epoxy mixtures.