Structural, spectroscopic and dielectric properties of thulium-doped laser-heated pedestal Ta 2 O 5 as-grown fibres were studied. Undoped samples grow preferentially with a single crystalline monoclinic structure. The fibre with the lowest thulium content (0.1at%) also shows predominantly a monoclinic phase and no intra-4f 12 Tm 3+ recombination was observed. For sample with the highest thulium amount (1.0at%), the appearance of a dominant triclinic phase as well as intraionic optical activation was observed. The dependence of photoluminescence on excitation energy allows identification of different site locations of Tm 3+ ions in the lattice. The absence of recombination between the first and the ground-state multiplets as well as the temperature dependence of the observed transitions was justified by an efficient energy transfer between the Tm 3+ ions. Microwave dielectric properties were investigated using the small perturbation theory. At a frequency of 5GHz, the undoped material exhibits a dielectric permittivity of 21 and for thulium-doped Ta 2 O 5 samples it decreases to 18 for the highest doping concentration. Nevertheless, the dielectric losses maintain a very low value.