Recombinant monoclonal antibodies (mAbs) have emerged as important therapeutic agents for the treatment of different diseases. This technology has been extended by the development of antibody drug conjugates (ADCs). ADCs are mAbs to which cytotoxic payloads are covalently attached, enabling the delivery of potent drugs to a specific target. This Application Note describes the analysis of an mAb (trastuzumab) and an ADC (trastuzumab emtansine) by size exclusion chromatography with online mass spectrometry (SEC-MS) using the Agilent 1290 Infinity II LC system with an Agilent 6530 Accurate Mass Q-TOF LC/MS system. We demonstrate that SEC is a straightforward way to introduce mAbs and ADCs into the LC/MS system, requiring little method optimization. From the MS data collected for the ADC, it was possible to calculate a drug-to-antibody ratio (DAR) of 3:2.