A test has been devised that can predict accurately the% crystallinity developed in a slag film formed between the steel strand and the mould. The% crystallinity in the test samples was determined and compared with that for slag films formed in the mould when casting with that flux. There was good agreement between the% crystallinity in the test samples and the slag films. It was found that additions of TiO2 and ZrO2 had little effect on the amount of crystalline phase formed, but additions of CaO caused a dramatic increase in the amount of crystalline phase formed. Various parameters (which can be calculated from chemical composition of the flux) have been plotted against% crystallinty in the test samples. It was found that the modified (NBO/T) ratio provided the best relation with% crystallinity for the various parameters tested. The recommended relation is% crystallinity= 141.1 (NBO/T)-284.0.