Neutron density and importance distributions, βeff/Λ and moderator temperature coefficients have been experimentally studied in connection with a central water gap of a light-water reactor having a water to UO2 volume ratio of 1.5. Thermal importance functions are determined from the reactivity weighting function, while the fast importance functions are measured by a 252Cf source introduction method. Values of βeff/Λ measured by pulsed-neutron experiments decrease from 202 sec−1 in the uniform core to 185 sec−1- for the core with a water gap of 6.7 cm effective radius. It is also shown that the water gap influences the moderator temperature coefficient to a considerable extent.