Tracers in drilling mud have been used during coring operations as a tool to quantify the degree of mud filtrate-invasion in the core. This paper describes the application of a deuterium oxide water-based mud tracer that allows for real-time analysis of the tracer concentration while the drilling operation is ongoing. This ensures that a uniform mud tracer concentration can be maintained during coring operations. By using a Cavity Ring-Down Spectroscopy (CRDS) technique, the tracer concentration offshore can be assessed within 30 minutes of sampling. The combination of deuterium oxide tracer and CRDS analysis constitutes the first HSE-compliant real-time tracing for offshore coring operations. This method was applied successfully on a Maersk Oil exploration well drilled in the North Sea. The mud tracer concentration was monitored throughout the coring operation and subsequently compared to the tracer concentrations found in the core material. It was concluded that mud filtrate invasion ranged from 26% to 62%.