Tracer tests from the Rosemanowes (Cornwall, UK) Phase 2B reservoir (1985 to 1991) are fitted to a model with adjustable parameters. The model is a weighted sum of three similar residence time density functions. The objective was to test if components from the decomposition of surface to surface tracer tests could be correlated to different flowing zones into the production well. The results revealed that a surface to surface tracer test is insufficient to distinguish between individual flow zones. Each flow zone is itself a combination of fluid with short, medium and long residence times inside the reservoir. The analysis also revealed that changes in the reservoir during circulation affected mainly the long residence time flow paths inside the reservoir, thus indicating a continuous evolution of the Rosemanowes reservoir during its circulation. This has implications for the thermal performance of the reservoir.