Reactivity effects of large voids in the reflector of the Pool Critical Assembly, an enriched-uranium, light-water-moderated and -reflected reactor, were investigated. The four reactivity effects studied were (1) variation of reactivity with void size, (2) variation of reactivity with void position on the core-reflector interface, (3) variation of reactivity with the distance between the void and the core, and (4) superposition of void reactivity effects. The variation of reactivity with void size and position on the core-reflector interface was correlated by a statistical weight correlation. An approximate theoretical method based on two-group diffusion theory was developed for calculating both the effect on reactivity and the effect on the neutron flux for a void covering one entire face of a reactor having a rectangular parallelepiped core. The calculated effects on reactivity and on the thermal-neutron fluxes were in reasonable agreement with experimental results.