The structure-property-relationships, thermal stability and flame retardancy of a DGEBA-DDS system containing various organo-phosphorus compounds as flame retardants is investigated. Three non-reactive (DOP-ethyl, DOP-ethylhexyl and DOP-cyanur) and one reactive (DOP-glycidyl) phosphorous compounds are added separately to the epoxy resin and the mixtures are cured with 4,4'-DDS in a substoichiometric ratio. The addition of such DOPO-compounds leads to improved flame retardancy at low phosphorus contents of about 2 wt.% (about 20 wt.% of additive) without significantly affecting other important properties such as fracture toughness (K-Ic) and glass transition temperature (T-g) of the matrix. Neither the type nor the amount of additive affects the fracture toughness of cured epoxies up to additive concentrations of between 18 and 24 wt.%. Furthermore, the loss in glass transition temperature of the cured resin can be correlated with the amount and chemical reactivity of the organo-phosphorus additive. The reactive DOP-glycidyl and the non-reactive DOP-cyanur additive are observed to maintain the highest glass transition temperature of the epoxy system mainly due to a higher extent of the cross-linking reaction. The results presented in this study highlight the potential of optimising the flame retardancy and the resulting physical and mechanical properties of epoxy systems for liquid composite moulding applications by varying the chemical structure of the organo-phosphorus compounds. (c) 2006 Springer Science + Business Media, Inc.