A three-part experimental study has been carried out to determine the effects of exposure to the spent nuclear fuel pool environment on two composite neutron-absorbing materials—one made in plate form and consisting of ∼72 wt% of B4C particles bonded together by ∼28 wt% of a phenol-formaldehyde polymer, and the other made in sheet form and consisting of ∼62 wt% of B4C particles bonded to both sides of a woven glass-fiber reinforcement by ∼19 wt% of the same polymer. The results of the mechanical and physical properties tests show that the two materials degrade somewhat differently in the spent fuel pool environment. In the case of the plate material, radiation-induced expansion and embrittlement of the polymer lead at doses ∼109 Gy to ∼1% linear expansion, with a concomitant ∼60% reduction in strength and stiffness and a somewhat enhanced susceptibility to contact damage. In the case of the sheet material, however, the presence of the relatively radiation damage-resistant glass-fiber reinforcement prevents such degradation of the polymer from causing either in-plane dimensional changes or loss of stiffness. Nevertheless, at doses ≳108 Gy, this latter material loses ∼60% of its ultimate tensile strength and becomes markedly more susceptible to the loss of B4C through contact damage. Parallel gas generation tests show that radiolytic decomposition of the polymer in air leads to evolution of H2 and a lesser amount of CO2 at rates of 0.4 to 0.5 X 10-7 cm3 [at normal temperature and pressure (NTP)]g-1 (of composite) Gy-1 and 0.2 to 0.4 X 10-7 cm3 (at NTP) g-1 (of composite) Gy-1 in the cases of the plate and sheet materials, respectively. Finally, the leachability test shows that about two-thirds of the 67 X 10-3% of the total boron content that is present on the surface of the B4C particles in the plate material as B2O3 is leached out during exposure to ∼3 X 108 Gy in deionized water at 308 K over a period of ∼ 100 days.