TlGaSe2 belongs to the family of ternary semiconducting chalcogenides with the chemical formula ABX2 where A abd B are metal atoms and X is a chalcogen atom. At room temperature paraelectric TlGaSe2 crystallizes in a monoclinic structure (C2/c). On cooling, two successive phase transitions take place: first at Ti = 120 K to an incommensurate phase which finally locks into a commensurate ferroelectric phase at TL = 110 K. The low temperature commensurate phase has the non-centrosymmetric space group C2. It is the loss of inversion symmetry at low temperature which leads the appearance of ferroelectricity.It is presumably caused by the small positional shift of Tl atoms in the a-b plane accompanied by the discontinuity in the axial ratio. The appearance of ferroelectricity has been interpreted as due to the stereochemically active electron lone pair configuration of the Tl+ ion. We determined the crystal structure of the commensurate phase in an old experiment. Here we propose to determine the structure of the incommensurate phase and also investigate the lock-in phase transition. For this we request 7 days of neutron beam time on D19.