Large-scale experiments examining spherical-flame acceleration in lean hydrogen-air mixtures were performed in a 64 m3 constant-pressure enclosure. Equivalence ratios ranging from 0.33 to 0.57 were examined using detailed front tracking for flame diameters up to 1.2 m through the use of a Background Oriented Schlieren (BOS) technique. From these measurements, the critical radii of onset of instability for these mixtures, on the order of 2-3 cm, were obtained. In addition, the laminar burning velocity and rate of flame acceleration as a function of radius were also measured.