We have succeeded in detecting UPD with respect to Pb underpotential deposition from Pb(ClO,), aqueous solution onto Ag substrate by using a photoacoustic technique. Photoacoustic (PA) measurements were carried out on a specimen of silver. The light source was an Ar ion laser. The light beam was chopped mechanically by a light chopper. Periodic pressure fluctuations, induced by thermal changes via nonradiative inactivation in the working electrode, were detected with a piezoelectric detector. The electrolyte used for plating or UPD, was an aqueous solution of NaClO,/Pb(ClO,),/HClO,. The current-potential curve showed the clear peak of Pb underpotential deposition at about -300 mV (deposition or dissolution) us see as reported by previous workers. On the other hand, PA signal-potential curves show the slight increase or decrease at the potential mentioned above. So this increase or decrease is thought to be caused by Pb UPD. The slow transformation phenomena of Pb adsorbates on Ag( 111) electrode surfaces has been reported previously. We have also succeeded in detecting this phenomenon at the potentials reported. This phenomenon was detected as a slight fluctuation of PA signal.