Longitudinal collective modes in liquid binary mixtures are investigated by molecular dynamics simulation. Li-Mg alloys at three concentrations and two isotopic liquid mixtures with mass ratios m(2)/m(1)=3 and m(2)/m(1)=6 have been simulated. The study is based on the calculation of both the dynamic structure factors and the longitudinal current spectra together with their corresponding dispersion curves. At low wave numbers the system shows a hydrodynamiclike behavior whereas at intermediate wave numbers the molecular nature of the system becomes apparent and the dynamic collective properties of light and heavy particles show a noticeable decoupling. Special attention is paid to the changes on the properties in the transition between the hydrodynamic and molecular regions. The existence of concentration modes in liquid mixtures is also discussed.