The High Flux Reactor (HFR) in Petten, The Netherlands, is a materials testing research reactor with an aluminium alloy (base material: 5154-O) vessel. The vessel wall is exposed to thermal and fast neutrons that induce transmutation and cause displacement damage in the metal lattice, respectively. The resulting effect of neutron bombardment is hardening of the aluminium alloy with a corresponding toughness decrease. Prediction of neutron radiation effects on the HFR vessel wall during its operational life is considered within a dedicated surveillance program SURP (an acronym for SURveillance Program). The surveillance program aims at monitoring the degradation of the vessel material at the most severely irradiated location by measuring the most relevant mechanical properties. In the SURP program, aluminium specimens placed in two specially designed experimental rigs are continuously irradiated. A number of surveillance specimens are periodically extracted and tested to prove fracture toughness of the reactor vessel material remains above the acceptable limit. The present paper outlines the most recent results of the HFR surveillance program, including fracture toughness properties trend as functions of thermal neutron fluences and of silicon content as well as microstructural analysis of changes in reactor vessel material due to neutron radiation.