The selection of the tracer used for residence time measurements is of great importance. It must be an inert substance that does not affect the physical properties of the reactor contents and can be measured easily. In particular, the viscosity and density of the tracer should be similar to the actual reaction solution. Furthermore, the tracer must be added at the same flow rate as the carrier solution is added to the reactor system under investigation. For the measurements presented here, THF was used as the carrier solution and toluene as the tracer. Though both solvents slightly differ in density and viscosity, THF is one of the most frequently used solvents for Grignard reagent syntheses and toluene represents a solvent which can also be used for Grignard reagent synthesis and which behaves inertly towards THF and the magnesium turnings but has a very similar density and viscosity to THF (table ).