Problems associated with electrochemical ac impedance measurements at frequencies up to 50 MHz are discussed in detail. Limitations in the impedance measurements arise from inductance and stray capacitance of leads connecting electrochemical cell to a measuring circuit. Theoretical considerations indicate that the best accuracy of measurements can be achieved when working with ultramicroelectrodes 2.5 x 10(-7) to 8 x 10(-6) cm(2). Construction of ultramicroelectrodes suitable for such measurements and a diagram of the electronic circuit is described. Preliminary results showing frequency dispersion of the double layer capacitance and solution resistance and the kinetics of adsorption of n-butanol are presented.