Abstract Use of fiber reinforced polymer (FRP) bars or grids instead of traditional steel reinforcements is an interesting application, especially to improve structural durability and extend the actual service life. As proved elsewhere by experimental and numerical results, a key aspect of the structural behavior of concrete slabs reinforced with FRP bars is the large reduction in both resistance and stiffness due to high temperatures. Thereupon six concrete slabs reinforced with glass FRP bars characterized by different values of concrete cover and anchorage length were recently tested in the fire situation. A comprehensive description of the experimental program and some significant experimental results are reported in a companion paper; the resistance of concrete slabs reinforced was showed strongly influenced by temperatures in the bars in the area directly exposed to fire but also in the anchoring area (not exposed zone). In this paper, at first, a detailed discussion of the experimental thermal readings is reported to give as a support to the remarks on mechanical response of the slabs reported in the companion paper. In particular the influence of concrete cover on temperature in exposed area and the temperatures in the area not exposed are examined to investigate the effectiveness of the anchor. The influence on the temperature in the bars of different concrete thermal properties is investigated. Furthermore, the influence of the thermal properties of the bars on the temperature field is evaluated. Finally, a simplified method for predicting temperatures of FRP bars in concrete slabs, obtained by considering the ISO standard fire curves is provided in the form of analytical relationships .Its reliability is showed by means of comparisons with the experimental results.