A series of 6-min microgravity combustion experiments of opposed-flow flame spread over thermally thick PMMA has been conducted to extend data previously reported at high opposed flows to almost two decades lower in flow. The effect of flow velocity on flame spread shows a square-root power-law dependence rather than the linear dependence predicted by thermal theory. The experiments demonstrate that opposed-flow flame spread is viable to very low velocities and is more robust than expected from the two-dimensional numerical model, which predicts that, at very low velocities (<5 cm/s), flame spread rates fall off more rapidly as flow is reduced. It is hypothesized that the enhanced flame spread observed in the experiments may be due to three-dimensional hydrodynamic effects, which are not included in the zero-gravity, two-dimensional hydrodynamic model. The effect of external irradiation was also studied and its effects were found to be more complex than the model predicted over the 0-2 W/cm(2) range. In the experiments, the flame compensated for the increased irradiation by stabilizing farther from the surface. A surface energy balance reveals that the imposed flux was at least partially offset by a reduced conductive flux from the increased standoff distance so that the effect on flame spread was weaker than anticipated.