The effects of thiourea as a catalyst poison on hydrogen absorption reaction (HAR) into Pd foil electrode in the under-potential deposition region have been investigated by using ac impedance spectroscopy combined with cyclic voltammetry and hydrogen permeation technique. The ac impedance measurements were carried out in the overpotential range of 0 to 0.22 V-RHE in 0.1 M NaOH solution containing various concentration of 0, 10(-8), 10(-7), and 10(-6) M thiourea. Measured impedance spectra were analyzed by using complex nonlinear least squares fitting method on the basis of faradaic admittance equations for the HAR under Impermeable boundary conditions. Based upon the measured thiourea concentration dependence of charge-transfer resistance, cyclic voltammogram, and steady-state permeation current density the transition of the HAR into the Pd foil electrode from diffusion controlled to inter-face controlled has been discussed at the concentration of 10(-7) M thiourea. The experimental results indicate that the poison thiourea added into the electrolyte impedes the HAR into the Pd electrode and hence binders the beta-PdH formation in the Pd electrode, in contrast to such metals as Fe, Ti, and Ni.