The solid and liquid phases of hydrogen, n-H-2 and HD, and methane, CH
D3 and CH3D, have been studied by dielectric measurements. It is found
that increasing deuteration has only a minor effect on the molar pola
rization of hydrogen, but that it leads to a significant decrease of t
he polarizabilities of the liquid methanes. The dipole moments of the
partially isotope substituted molecules have been determined in the so
lid state. For HD a local field corrected moment of 0.78 mD is found,
for CHD3 and CH3D the moments are 4.3 and 4.1 mD, respectively.