We demonstrate anomalous gaseous field ionization and field desorption on branching intrinsic silicon nanowires grown by a two-step VLS technique. Field ionization and desorption I-V curves of argon, nitrogen, helium, and ammonia, were recorded individually within a wide pressure range (10-7 to 10 Torr). Field ionization initiated at sub volt was followed by field desorption at about 7 - 38 V (applied field of ∼ 7×102 to 3.8×103 V/cm). Such voltages are three orders of magnitude smaller than the applied voltages required to generate field ionization on sharp metallic tips having the same tip curvature. The measured I-V curves were pressure dependent. Low voltage filed ionization and desorption phenomena were attributed to the combination effects of geometrical field enhancement on the apex of nanoscale silicon branches, field penetration, increased tunneling critical distance, band gap widening due to quantum confinement, and the surface states formed by the catalyst. The results presented herein suggest that gold terminated branching silicon nanowires could be strong candidates in building low power gas ionization sensors useful in highly selective detection of gases with low adsorption energies.