Refractive index gratings in planar LiNbO3:Ti:Fe waveguides are thermally fixed during hologram recording at elevated temperatures. Different guides are fabricated by titanium indiffusion using iron-doped and nominally pure y-cut LiNbO3 substrates and characterization is performed by dark-mode spectroscopy. The refractive index modulation of gratings written and simultaneously fixed at 180°C is investigated as a function of propagation depth, titanium and iron concentration. The experimental results are compared with those obtained for unfixed holograms recorded at room temperature. © Springer-Verlag 1998.