The phase-separation behavior of blends consisting of 4,4‘-diaminodiphenylsulfone (DDS) cured diglycidyl ether of bisphenol A (DGEBA) with polyoxymethylene (POM) was investigated by means of small-angle laser light scattering (SALLS), optical microscopy (OM), and scanning electron microscopy (SEM). Curing results in reaction-induced phase separation (RIPS). Blends with different amounts of POM at curing temperatures above (180 °C) and below (150 and 145 °C) the melting temperature of POM (T_m = 168 °C) are considered. At an isothermal curing temperature of 180 °C, all blends demix according to a spinodal decomposition mechanism with the clear signature of viscoelastic effects at specific POM contents. Furthermore, curing leads to changes in the refractive indices of the phases, occasionally resulting in an apparent phase dissolution, which disappears again at the onset of secondary phase separation. Crystallization after curing at 180 °C can only be induced by cooling and is discussed using CTT diagrams for the blends with 10 and 20 wt % POM. Curing at 150 °C results in RIPS, preceding isothermal crystallization at the cure temperature. Curing at 145 °C induces isothermal crystallization in the homogeneous blend, followed by RIPS between the growing spherulites. At 145 °C a gradient of phase-separated structures is observed, when the growing spherulites sweep through the amorphous matrix where RIPS is in progress.