TlCoCl3 and Tl FeCl3 undergo structural phase transition at T1= 165K, respectively T1= 179K and T2=75 K, respectively 79 K. The unit cell stay hexagonal but the basic unitcell of a x a x c are enlarge by a factor aV3, respectively 4a by lowering the temperature.The proposal concerns the determination of the details of the low temperature structure of both compound at 40 K. This structure is most likely modulated. The modulation removes the structural disorder found in the aV3 phase. We propose to obtain structural data over the complete reciprocal space. At T=40 K and 85K for TlFeCl3 and TlCoCl3.The magnetic structure and ordering of TlCoCl3 will be studied at 2K and 30K.We expect to observe diffuse nuclear scattering based on the 4a x 4a x c super cell to be present in the data set of the aV3 x aV3 xc unit cell at 85K. The magnetic behaviour will be clarified by these results