Perturbation of the 2201-cm Ϫ1 absorption line of the Al-H complex in Si by uniaxial stress is shown to be consistent with a stretching mode of a center with an effectively trigonal symmetry. Quantitative analyses of the effects of the stresses and of varying the temperature reveal the presence of a very-low-frequency transverse mode which transforms as the E irreducible representation in the trigonal symmetry. Its frequency is 64 cm Ϫ1 in the ground state of the stretching mode ͑representation A 1 ͒. An A 1 ϩE combination mode, observed only under stress, lies 24 cm Ϫ1 above the excited state of the A 1 mode. Upon substituting D for H, the frequencies of the stretching and transverse modes are reduced by approximately a factor of 1/&, as expected for vibrations which predominantly involve the H ͑D͒ atom. The symmetries of the center and of its modes are unchanged by the isotopic substitution. The Al-H axis may be preferentially aligned along one of the ͗111͘ axes by an applied stress. The barrier to reorientation is ϳ370 meV. ͓S0163-1829͑96͒03139-6͔