Guided-mode resonances (GMR) manifest as narrow-linewidth transmission or reflection bands when waves diffracted from a grating couple to propagating modes in an adjacent dielectric substrate [1]. Recently, such structures were proposed for the design of narrowband filters operating at terahertz (THz) frequencies, suitable for applications in telecommunications, radar science, or imaging [2].Here, we investigate both theoretically and experimentally the properties of GMR in terahertz filters based on metallic frequency-selective surfaces (FSS), which are patterned via photolithography on thin films of the lowloss cyclo-olefin polymer Zeonor. Filters with very high quality factors are designed and their spectral response is studied under different conditions, such as oblique incidence, rotation of the polarization plane, and bent configurations. Contrary to the filtering response of standard free-standing FSS-THz filters, we observe extensive tunability of the GMR frequencies by means of mechanically rotation, as well their suppression by bending the flexible substrate [3, 4].