An experimental study on stability limits of premixed hydrogen-air flames in planar microcombustors (H=1 mm and 1.5 mm) partially filled with porous medium is carried out, focusing on the effects of combustor sizes and filling conditions. Critical conditions for blow-off, flashback, and breaking through the porous medium are experimentally measured. The blow-off limits are nearly independent of combustor sizes and filling conditions, while the flashback limits are strongly influenced by the combustor size and the filling conditions. Critical values for breaking through are identified with two different methods, and it is shown that standing combustion waves are settled over a range of velocities, instead of a fixed value of filtration velocity, which is considered an important characteristic of microcombustion. Most results can be explained by the classic boundary velocity gradient theory by von Elbe and Lewis, and thus the validity of the theory to the present channel spacings is confirmed. (c) 2015 American Institute of Chemical Engineers AIChE J, 61: 2571-2580, 2015