The five isotherms limiting the title quaternary system have been investigated as a first step towards the study of this system at 25-degrees-C and 1 bar. Results are discussed and compared with bibliographic data, when available at 25-degrees-C. The use of the 25-degrees-C isotherm of this system (when completed) and the 75-degrees-C isotherm that we have previously reported will lead to the most practical conditions for manufacture of P-K fertilizer. Anhydrous reciprocal ternary system : H2SO4-H3PO4-K2SO4-K3PO4. Solid phases : KHSO4.H2SO4, KHSO, KH2PO4.KHSO4, KH5(PO4)2, H3PO4, and a solid substituting solution Kg(H2PO4)1+x(HSO4)7-x 0 less-than-or-equal-to x less-than-or-equal-to 1, continuous between limiting compositions KH2PO4.7KHSO4 and KH2PO4.3KHSO4. In the region near pure H2SO4, a double salt line (KHSO4.H2SO4 and KHSO4) can occur, if the H2SO4 used is not perfectly anhydrous. Ternary system : K3PO4-H3PO4-H2O. Our results are in good agreement with a bibliographic data established at 25-degrees-C in the range 0-73 wt% H3PO4. For compositions greater than 94.31 wt% H3PO4, a crystallisation field of H3PO4 was delimited. Ternary system : K2SO4-H2SO4-H2O. Solid phases : K2SO4, K2SO4.KHSO4, K2SO4.7KHSO4.H2O, KHSO4 and KHSO4.H2SO4. A phenomenon of retrosolubility as function of the water content, similar to the one previously reported at 75-degrees-C, was observed. Ternary system : K2SO4-K3PO4-H2O. Solid phases : K2SO4 and K3PO4.7H2O. Ternary system : H3PO4-H2SO4.H2O. Only one crystallisation area was observed, corresponding to solid H3PO4.