Monosulfate (3CaO·(AlxFe1x)2O3·CaSO4·12H2O) is an AFm phase that can be formed during the hydration of cement. Fe-containing monosulfate and (Al,Fe) mixed monosulfate were synthesized and characterized. Fe-monosulfate is composed of a positively charged main layer [Ca2Fe(OH)6]+ and a negatively charged interlayer [1/2SO4·3H2O]−, crystallizes in the trigonal symmetry, and is isotypic with Al-monosulfate. The solubility product at 25°C was determined to be −31.57. The formation of solid solution due to Al-Fe substitution in the main layer structure of monosulfate was observed. Based on the evolution of the unit cell parameters and the thermodynamic investigations, a presence of solid solution from 0.0 to 0.45 Al/(Al + Fe) ratio and a miscibility gap in the range 0.45 < Al/(Al + Fe) ratio <0.95 is suggested.