Abstract: A solution synthesis method of producing K₃AlF₆-KAlF₄ eutectic flux with particle size ranging from 0.1μ to 5μ in diameter without grinding, has been made successfully by the reaction of Al(OH)₃ with KOH solution followed by acidizing with HF. No free KF was found in the product. The brazing mechanism of this type of flux has also been investigated. Evidence indicates Si contamination is easily carried into the product from silicon containing holder^wall in any kind of synthesis method. However, the dissolved Si, though often neglected, plays the role of interfacial surfactant during brazing. Very pure potassium fluoaluminate flux sheer free from Si, on the other hand,is lacking in brazing activity. The isothermic thermogravimetry data show that the SiF₄ decomposed from the K₂SiF₆ addition or produced by the reaction of flux with holder-wall may be stabilized by AlF₃. Perhaps this is due to the formation of certain compounds like mKF·nAlF₃·pSiF₄ in the flux.