We report the study of dielectricproperties of amorphoussemiconducting YBCO thinfilms measured in a broad frequency range, from 40 Hz to 2 GHz and in the 210 to 430 K temperature range, using a coaxial discontinuity technique. At all temperatures, dielectric permittivity and conductivity spectra exhibit typical features of dielectric relaxation processes. A first relaxation at low frequency (300 Hz-2 kHz), observed only at high temperature, might be attributed to interfacial effects. Polarization in the grains of the YBCO thinfilms could lead to the second relaxation observed at higher frequency (800 Hz-660 kHz). The relaxation frequencies increase with temperature and follow a thermally activated behavior of the Arrhenius-type. Optical near-infrared response of YBCO bolometers exhibits a corner frequency around 40 kHz. Above this frequency, the thermal diffusion is localized inside the YBCO thinfilm. This frequency is in good agreement with the relaxation frequency attributed to YBCO grains in the thinfilm, i.e. around 40 kHz at 300 K.