A new MEMS-based density and binary gas concentration sensor is discussed. The micromachined sensor is made using a resonating silicon tube. Gas density and concentration test data over pressure for air, hydrogen, nitrogen, argon, methane, carbon dioxide, sulfur hexafluoride and gas mixtures are presented. The measurement of the density low pressure (101–400kPa) hydrogen is demonstrated. Coupling binary gas concentration and density measurements of low pressure, light gases like hydrogen and methane opens up new sensing applications like fuel cells, energy management, chemical and semiconductor processing.