Nearly single-phase superconducting (Hg 0 . 7 , Tl 0 . 3 ) 2 Ba 2 (Ca x ,Y 1 - x )Cu 2 O 8 - 2 δ samples (x = 0.1, 0.2, 0.4 and 0.6) were prepared by a high pressure synthesis technique. The crystal structure of these samples was determined by X-ray Rietveld refinement. This crystal belongs to a tetragonal system and has a space group of I4/mmm with a = 3.86 and c = 29.0 . The unit cell parameter a decreases and c increases with increasing x. The expansion of the c-axis is found to mainly result from the increase in the inter-planar distance between the Ba-Hg planes. The present refinement does not show evidence for the substitution of Cu in the (Hg,Tl) site.