In any sort of rubber toughening of thermosets, there are two principal influences on toughness: Morphology of the elastomer particulate phase and toughenability of the matrix resin. These two factors are difficult to separate experimentally. A new approach is presented which is based on large numbers of data. By analysing property-property-relationships an attempt is made to identify generalizable rules. A tri-block copolymer (PCL-PDMSi-PCL) is analysed in detail: By varying the molecular weights of the compatible and incompatible blocks in two different epoxy resins the influence of morphology on toughness is studied. Linear (T-g and rubbery plateau modulus) and non-linear (yield stress and post-yield behaviour) matrix properties are examined for a larger number of resin/curing agent combinations toughened with three different toughening agents in order to elucidate which matrix property controls toughenability. The same resin/curing agent array was used to study the influence of matrix polarity on morphology and properties for one PCL-PDMSi-PCL tri-block-copolymer.