Abstract
We investigate the ability of primordial gas clouds to retain molecular hydrogen (H2) during the initial phase of the reionization epoch. We find that before the Strömgren spheres of the individual ionizing sources overlap, the UV background below the ionization threshold is able to penetrate large clouds and suppress their H2 abundance. The consequent lack of H2 cooling could prevent the collapse and fragmentation of clouds with virial temperatures Tvir ≲ 104 K (or masses ≲108 M☉[(1 + zvir)/10]-3/2). This negative feedback on structure formation arises from the very first ionizing sources and precedes the feedback due to the photoionization heating.