Context :

Aims : We show that the line broadening in quiescent solar prominences is mainly due to non-thermal velocities .

Methods : We have simultaneously observed a wide range of optically thin lines in quiescent prominences , selected for bright and narrow Mg b emission without line satellites from macro-shifts .

Results : We find a ratio of reduced widths , \Delta \lambda _ { D } / \lambda _ { 0 } , of H \gamma and H \delta of 1.05 \pm 0.03 , which can hardly be attributed to saturation , since both are optically thin for the prominences observed : \tau _ { \gamma } \leq 0.3 , \tau _ { \delta } \leq 0.15 .
We confirm the ratio of reduced widths of He 4772 ( triplet ) and He 5015 ( singlet ) of 1.1 \pm 0.05 at higher significance and detect a width ratio of Mg b _ { 2 } and Mg 4571 ( both from the triplet system ) of 1.3 \pm 0.1 .

Conclusions : The discrepant widths of lines from different atoms , and even from the same atom , can not be represented by a unique pair [ T _ { kin } ; V _ { nth } ] .
Values of T _ { kin } deduced from observed line radiances using models indicate low temperatures down to T _ { kin } \approx 5000 K. Non-thermal velocities , related to different physical states of the respective emitting prominence region , seem to be the most important line broadening mechanism .