Changes in axial stress and refractive index have been measured as a function of the total UV-irradiation fluence during the formation of Bragg gratings in B/Ge- and Sn/Ge-codoped core fibers. For the B/Ge-codoped core fiber, a fast development of the type-I grating with a significant increase in refractive index and tensile stress in the fiber core is followed by a slow growth of the type-IIA grating with a gradual decrease of refractive index and tensile core stress. A strong UV-induced reduction of the core stress and a domination of the type-IIA grating are observed for the Sn/Ge-codoped fiber. The index changes of the type-I and type-IIA gratings are believed to be governed by the compaction of the UV-irradiated core network and its dilation, respectively.