A CMOS-compatible process is proposed for the fabrication of thermopiles with high sensitivity in the 3–5 μm window, suited for use in non-dispersive infrared (NDIR) or photo acoustic (PA) gas sensors. Since CMOS passivation layers present low infrared absorption at those wavelengths, an interferometric absorbing layer based on thin metals with a dielectric spacer is adopted and embedded in the fabrication process. Different CMOS-compatible thermoelement designs are investigated and compared in terms of performance, considering Al/n-polysilicon, Al/p-polysilicon and p-/n-polysilicon solutions. Electrical tests on the fabricated devices indicate a noise equivalent power of about 0.76 nW for 1 mm2 wide p-/n-polysilicon thermopiles, which makes such devices applicable in NDIR or PA-based sensing of relevant gases like CO, CO2, N2O or CH4 having absorption lines in the 3–5 μm wavelength range.