A new method for determining the steam, or void, fraction in a boiling-water out-of-pile test section has been theoretically analyzed and experimentally verified with air voids at room temperature. The method utilizes the combined moderation and attenuation of neutrons from an Sb-Be source by the boiling water in which boron has been dissolved. It is found that the method is applicable to multi-rod test sections with void fractions up to 25% or more, and is sensitive enough to detect void fractions as small as 3% or less.