The dynamic collective properties of the liquid Li0.7-Mg0.3 alloy are studied by molecular-dynamics simulation. The dynamic structure factors and longitudinal current correlations at wave vectors between the hydrodynamic and kinetic regime (0.2,k, 4A 21 ) are analyzed. In order to discuss the influence of the mass difference between particles on the longitudinal modes, the same alloy, except that the mass of the heavy atoms was increased by a factor of 10, was simulated. The resulting properties are compared with those of the ordinary Li0.7-Mg0.3 alloy. It is shown that at wave vectors prior to the hydrodynamic region both fast and slow longitudinal modes of kinetic character propagate through light and heavy particles, respectively. In the hydrodynamic limit fast and slow modes merge into a single acoustic mode. It is corroborated that fast propagating modes in disparate mass liquid mixtures have a kinetic character. The number-number and concentration-concentration time correlation functions were also determined. The former shows a behavior analogous to that of the corresponding function in one-component liquids. The second reflects the existence of propagating concentration modes.