A device for measuring the streaming potential of fibers and particles in suspensions, in particular a device for determining the zeta-potential of the particles of aqueous suspensions containing fibers and particles, by measuring the streaming potential, and by subsequently computing the zeta-potential with an empirical formula. A periodic pressure curve is generated by arranging in the device for measuring the streaming potential at least two vacuum vessels, with each vessel having a different constant reduced pressure. The vacuum vessels are permanently set to a different vacuum and alternatingly connected via a valve to the measurement cell with the fiber plug. The required reduced pressure in the two vacuum vessels is maintained by using at least one small, low-capacity vacuum pump and a valve controller.