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A thermodynamic study was performed on diffusion growth of Ga 3+ -doped LiTaO 3 (LT) thin film for integrated optics. Some Ga 3+ -doped LT thin films were grown on LT surface by in-diffusion of homogeneously coated Ga 2 O 3 film at the temperature range of (1273 to 1473) K. After growth, the refractive indices at Ga 3+ -doped and un-doped surface parts were measured by prism coupling technique and Li composition there was evaluated from the measured refractive indices. The results show that Ga 3+ dopant has small effect on the LT index. Li 2 O out-diffusion is not measurable. The Ga 3+ profile in the grown thin film was analysed by secondary ion mass spectrometry. It is found that the grown Ga 3+ ions follow a complementary error function profile. A thermodynamic model for Ga 3+ diffusion growth is suggested and verified experimentally. From the measured Ga 3+ profiles, some thermodynamic parameters were obtained. These include diffusivity, diffusion constant, chemical activation energy, solubility, solubility constant and enthalpy of solution. These parameters are crucial to design and growth of a desired Ga 3+ -doped LT thin film for integrated optics. Comparison shows that Ga 3+ grows in LT two orders faster than Ti 4+ because of its smaller radius. Ga 3+ diffusivity/solubility in LT is one order/1.5-fold lower than in LiNbO 3 because Ta has a larger atomic mass and hence a lower mobility than Nb. [ABSTRACT FROM AUTHOR]