By using a piezoelectric quartz crystal (PQC) with one separated electrode as an optically-transparent electrode (OTE), a new technique of absorption spectroelectrochemistry (SEC) in combination with piezoelectric quartz crystal detection is proposed. An equation connecting the absorbance variation due to the consumption of the colorful solution species to the frequency shift is derived. Cu(II) in glutamic acid + KNO3 supporting electrolyte is used to test this new technique. The results show that this technique can provide substantial in situ information in the investigation of the electrochemical process.