Electrode-electrolyte interface (EEI) analysis is of great significance in the investigation of electrochemical (EC) processes. Current ex situ surface techniques provided rich information of the structural/chemical composition of the EEI. However, real-time and in situ monitoring of the EEI is in great demand for a better understanding of the dynamic changes of the redox systems. Mass spectrometry (MS) is a useful technique for mass-resolved analysis of various analytes with the capability of on-line measurement. In this review, we give a brief overview of the development of online EC-MS techniques. Recent advances of well-established EC-MS techniques, such as differential electrochemical mass spectrometry and EC-electrospray ionization mass spectrometry, and their applications are summarized. Specifically, some newly designed EC reactors coupled with state-of-the-art mass spectrometry for the detection of short-lived intermediate species are highlighted.