Hydrogen evolution and absorption during Zn electrodeposition from zinc sulfate electrolytes containing polyethylene glycol (PEG), gelatin, and thiourea (TU) was studied using an electrochemical quartz crystal microbalance (EQCM). The results obtained indicate that PEG and gelatin act as inhibitors of hydrogen absorption on the deposited Zn and that TU acts as an inhibitor of hydrogen evolution but yet promotes hydrogen absorption on the Zn deposits. Gelatin suppressed hydrogen reduction most effectively. EQCM proved to be a useful in situ tool for investigating the effects of hydrogen evolution and absorption on metal deposition and for distinguishing between the cathodic charge for Zn deposition and hydrogen reduction reaction. (C) 2003 The Electrochemical Society.