The adsorption of deutero-methane (CD₄) in Cu ₃(btc)₂ (HKUST-1) was investigated at 77 K using high-resolution neutron powder diffraction. Rietveld refinement of the neutron data revealed a sequential filling of the rigid framework at distinct preferred adsorption sites, and showed the importance of open metal sites even for non-polar molecules such as methane. Four main adsorption sites were identified, located inside the small and two larger pores of the framework. The shorter distances between the CD₄ center and the pore wall atoms are covering a range from 3.07 to 3.547 . The maximum occupation of 170 CD₄ molecules per unit cell, estimated from the refined occupancy of the adsorption sites, is close to the value estimated from volumetric adsorption isotherms at 77 K (176 molecules per cell). Molecular simulation gave further insight into the adsorption mechanism.