Abstract The growth of III-nitrides on the ferroelectric materials lithium niobate (LN) and lithium tantalate (LT) via molecular beam epitaxy (MBE) using rf plasma source has been investigated. We have found that gallium nitride (GaN) epitaxial layers have a crystalline relationship with lithium niobate (tantalate) as follows: (0 0 0 1) GaN || (0 0 0 1) LN (LT) with [10−10] GaN || [11−20] LN (LT). The surface stability of LN and LT substrates has been monitored by in situ spectroscopic ellipsometry in the vacuum chamber. Three different temperature zones have been discerned; surface degas and loss of OH group (100–350 °C); surface segregation/accumulation of Li and O-species (400–700 °C); surface evaporation of O-species and Li desorption (over 750 °C). However, LT shows only surface degassing in the range of 100–800 °C. Therefore, congruent LN substrates were chemically unstable at the growth temperature of 550–650 °C, and therefore developed an additional phase of Li-deficient lithium niobate (LiNb 3O 8) along with lithium niobate (LiNbO 3), confirmed by X-ray diffraction. On the other hand, LT showed better chemical stability at these temperatures, with no additional phase development. The structural quality of GaN epitaxial layers has shown slight improvement on LT substrates over LN substrates, according to X-ray diffraction. Herein, we demonstrate AlGaN/GaN heterostructure devices on ferroelectric materials that will allow future development of multifunctional electrical and optical applications.