A feedstream jet intermittency phenomenon and its onset conditions in a continuous stirred tank reactor are detected for the first time using a laser sheet induced fluorescence visualisation technique. The aim of the present investigation is to determine the operating conditions for which the intermittency phenomenon occurs in stirred tank reactors The jet intermittency parameters studied are: stirrer type (Rushton and 45 degrees pitched blade turbines, profiled Mixel TTP propeller), stirrer rotational speed, fluid viscosity, feeding rate and the feed point location in the tank. In the case of the Rushton turbine stirred tank reactor, diagrams of intermittency occurrence are reported. These diagrams represent the feeding jet velocity as a function of specific power input or local fluid velocity in the tank. It was not possible to derive from these diagrams any valuable correlation. In order to predict and scale-up the occurrence of intermittency in industrial stirred reactors, dimensionless correlations characterising intermittency occurrence in these devices have been modelled for the three types of stirrers. The dominant variables determining the occurrence of intermittency is the ratio of the jet velocity v(J) to the tip velocity v(tip).