Measurements of homogeneous nucleation rates of two binary vapor mixtures of water and alcohol carried in nitrogen have been performed with a new instrument for nucleation and droplet growth studies, the piston expansion tube (pex-tube). The pex-tube subjects the vapors to short supersaturation pulses generating a limited number of nuclei which grow into monodisperse droplets. The droplets are detected in number and size by Mie lightscattering. The results provide nucleation rates between 10(5) and 10(9) cm(-3) s(-1) at constant temperature as a function of the vapor phase activities a(1) and a(2) of water and alcohol, respectively. Comparison with published experimental data exhibits good agreement.