In order to develop a fully CMOS compatible CO2 micro gas sensor based on evanescent field absorption in the mid-infrared, it is essential to first characterize separately the various components of the sensor: the optical waveguide structures, the light source and the detector. The experimental results provide a feedback for the design and fabrication steps, and can be used for the design of improved structures. First, the presentation will give a short introduction about the sensing principle and the experimental methods employed for the optical and electrical characterization of the sensor components. Second, the results of the characterization will be presented, with focus on: (1) Optical characterization of losses in slab waveguide structures. (2) Electrical characterization of the on-chip light source and detector. Finally, a first proof-of-principle measurement of CO2 from structures containing both electrical and optical elements will be presented.