The boundary curves and quaternary invariant points of the reciprocal system H3PO4-K2SO4-H2SO4-K3PO4-H2O were investigated at 25 degrees C and 1 bar by chemical analysis of solutions in equilibria with two or three salts, respectively. Thirteen compounds occur in this system. Only ten of them have crystallization areas inside Janecke’s prism. These are K2SO4, K2SO4 KHSO4, K2SO(4).7KHSO(4).H2O, KHSO4, KHSO4.H2SO4, KH2PO4.KHSO4, KH5(PO4)(2), KH2PO4, H3PO4 and a solid solution K-8(H2PO4)(1+x)(HSO4)(7-x) (0 less than or equal to x less than or equal to 1) of substitution type, which is continuous between the two limiting compositions KH2PO4.7KHSO(4) and KH2PO4.3KHSO(4). The crystallization fields of KH2PO4.2K(2)HPO(4).H2O, K2HPO4.3H(2)O and K3PO4.7H(2)O are very close to the solubility curves of the ternary system K3PO4-H3PO4-H2O. The results are discussed and compared with bibliographic data.