This International Standard specifies a reduced molybdosilicate spectrophotometric method for the determination of the silica content of natural and artificial cryolite and of natural and synthetic materials having a molar ratio (NaF/Al F3) between 3 and 1,7 approximately. The method is applicable to products the phosphorus(V) oxide (P2O5) content of which does not exceed 0,02 % (m/m).