Low temperature x-ray study of geometrical isotope effect in hydrogenated and deuterated single crystals of both /spl kappa/-(BEDT-TTF)/sub 2/Cu(NCS)/sub 2/ and /spl kappa/-(BEDT-TTF)/sub 2/Cu[N(CN)/sub 2/]Br -(bedt-ttf)2cu[n(cn)2]br.

Summary form only given. An isotope effect by deuteration of the terminal ethylene on the lattice parameters has been studied for quasi-two-dimensional organic superconductors, /spl kappa/-(BEDT-TTF)/sub 2/Cu(NCS)/sub 2/(Tc=9.5K) and /spl kappa/-(BEDT-TTF)/sub 2/Cu[N(CN)/sub 2/]Br (Tc=11.5K) by four-circle X-ray diffraction measurements down to 13 K. For both salts, the deuteration effect is found in the lattice parameter normal to the zigzag polymeric chain of anions, while the interlayer spacing and the lattice parameter parallel to the chain are little changed within experimental error. The so-called geometrical isotope effect is discussed in terms of the isotope (deuteration) shift of Tc. The lattice anomalies around the conformational ordering temperature 80-100K, which have already been revealed in our previous work [1, 2], are also discussed, focusing on the difference between these two salts.