Abstract The break temperatures of mould fluxes are important since they help to control the horizontal heat transfer and lubrication between the steel shell and the mould, and consequently affect occurrences of longitudinal cracking and sticker breakout in continuous casting. Break temperatures T br have been determined for both steady state and dynamic measurement of the viscosity, and equations relating T br to chemical composition have been obtained for both cases. It has been found that T br can be affected by (i) cooling rates and (ii) fluorine losses during the measurements.