A geopolymer kaolin phosphate binder has been synthesized based on metakaolin obtained from natural kaolin (750 o C) and 40 % orthophosphoric acid. The choice of aluminum silicate Al2O3·2SiO2 was due to the potential possibility of introducing a valuable component – mullite 3Al2O3·2SiO2 into the matrix in the process of heat treatment of a composite made on this binder using ceramic technology. Using IR spectral analysis (bands 1250-1030, 1113-1092, 810-609, 744 cm-1), the formation of Si–O–P, Al–O–P, Si–O–Al bonds characteristic of geopolymers was confirmed. In the process of the binder burning, various aluminum phosphates were formed, in particular, Al(PO3)3 and AlPO4, as well as silicophosphate SiP2O7. Starting from 900 o C, mullite appeared. By chemical method (by the mass of the residue not dissolved in hydrofluoric acid), the content of mullite formed in the temperature range of 900-1500 o C was determined. With further use of this binder in the synthesis of hightemperature composites, for example, for refractory purposes, the mullite phase will play a positive role in the composition of the solidified matrix, since mullite has high refractoriness, chemical and thermal resistance.