Crack-tip deformation has been investigated in pre-cracked samples of core-shell rubber particle modified epoxy resins with different degrees of crosslinking. Light cross linking resulted in cavitation of the modifier, the formation of croids and extensive crack-tip yielding, whereas these deformation mechanisms were suppressed at room T in highly crosslinked specimens. A zone of particle debonding was observed in these latter at high T, but this appeared detrimental to the toughness. Indeed it was possible to increase the toughness in this case by oxygen plasma treatment of the modifier particles.