By means of inelastic neutron scattering we measure the dynamic structure factors S(k,omega) of three dense mixtures of neopentane and Ar-40 (invisible for neutrons) at 323 K for wave numbers 46 nm(-1) and n(neo)=2.7 nm(-3), omega(H)(k) shows a behavior reminiscent of ''de Gennes narrowing.'' For n(neo)=3 nm(-3), with C(CH3)(4), the (incoherent) S(k,omega) can be described with two Lorentzians, due to translational and rotational motions of single neopentane molecules, yielding the self-diffusion coefficient D-s=9X10(-9) m(2)s(-1) and the rotational diffusion coefficient D-r=0.8+/-0.3 ps(-1). The results are consistent with kinetic theory calculations.
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