Results are presented for the velocity of ultrasound in liquid ⁶Li-⁶Li alloys of composition 4.5, 49.7 and 99.9 at.% Li for temperature up to 700 degrees C. At the melting point the ratio of the velocity of sound in ⁶Li to that in ⁷Li was found within experimental error to equal (M₇/M₆)¹2/, the result expected for classical liquids which differ only in the isotopic mass M. In the alloy of 49.7 at.% ⁷Li the sound velocity exceeded 0.6% the value expected for a thermodynamically ideal alloy. This result is discussed in terms of the theoretical treatment by Parrinello, Tosi and March (1974) of collective excitations in binary isotopic fluids.