Ultrafast photothermal (PT) and photoacoustic (PA) investigation of a metal--solution interface was performed by using subpicosecond time-resolved transient reflectivity (TR) measurement. A polycrystalline platinum film (thickness 30 nm) vapor-deposited on a glass substrate and a polycrystalline platinum disk were investigated in air and in a variety of solutions with and without electrochemical potential control. Transient reflectivity change during and after pulsed laser illumination was measured with a high-precision (<10[sup -6]) and high temporal resolution of the pulse width limit (pulse duration 240 fs FWHM). It was found that, in a picosecond time scale, the TR response showed electrolyte dependence as well as electrochemical potential dependence [A. Harata, T. Edo, and T. Sawada, Chem. Phys. Lett. 249, 112--116 (1996)]. These results suggest that interface structure in a molecular scale affects the ultrafast PT/PA phenomena occurring just at the interface. Also observed were ultrahigh-frequency longitudinal acoustic waves ringing in the ultrathin platinum films sandwiched between the glass and the solution. The frequency observed was up to 300 GHz for the sample in air and a strong acoustic damping in the solution was observed.