The microstructures of novel phosphoric irons have been studied by metallography in order to understand the phase distribution and phase transformations in the iron-rich end of the Fe-P phase diagram. Both Nital etchant and Oberhoffer’s reagent were effective in revealing dual-phase microstructures of phosphoric irons that were heat treated in the (α + γ) region. The phase distribution in dual-phase microstructures was analyzed by quantitative metallography and related to the phase diagram. The morphology of austenite depended on phosphorus content and heat-treatment temperature. The microsegregation of phosphorus and phosphorus diffusivity was studied as a function of the temperature and soaking time of the specimens. The beneficial effect of the heat treatment of phosphoric irons in the dual-phase region of the Fe-P phase diagram on mechanical behavior has been confirmed by tensile testing.