The phase stability in the pseudoquaternary CaO-SiO2-Al2O3-Fe2O3 system has been studied over the temperature range of 1700degrees to 1360degreesC. The samples were prepared so that the mean chemical compositions were distributed between the end members of P2O5-bearing Ca2SiO4 (C2S(ss)) and Ca2AlFeO5. They were examined by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. Above 1378degreesC, alpha-C2S(ss) and the liquid coexisted in equilibrium. Based on the temperature dependence of the chemical compositions of the two phases, the phase diagram was established in part of the pseudobinary Ca2SiO4-Ca2AlFeO5 system. The Ca2AlFeO5 contents of alpha-C2S(ss) and liquid steadily increased from 3.4 to 5.2 and from 51.6 to 80.0 mass%, respectively, as the temperature decreased from 1700degrees to 1380degreesC. At temperatures between 1378degrees and 1368degreesC, the three-phase mixture of alpha(H)(')-C2S(SS), ferrite, and liquid was stable. When alpha-C2S(ss) and the liquid above 1378degreesC were cooled, both the remelting (metatectic) reaction of alpha-C2S(ss) and the fractional crystallization of the liquid simultaneously occurred in the narrow temperature range of 1378degrees to 1368degreesC. In the latter reaction, the crystallized ferrite became enriched in Fe2O3 with respect to Al2O3; thus the relative composition of the differentiated liquid was reversed. During further cooling below 1368degreesC, the aluminate was crystallized out of the liquid. The resulting phase compositions (i.e., alpha(H)'-C2S(ss), ferrite, and aluminate) were therefore distinct from those which were thermodynamically most stable (i.e., alpha(H)'-C2S(ss) and ferrite).