The evolution of the microstructure, microchemistry, and magnetic properties during slow cooling of melt-spun Sm(Co,Fe,Cu,Zr)z magnets was investigated. It was found that uniform cellular and lamellar structures are formed upon isothermal aging the as-spun ribbons at 850 °C for 3 h, without subsequent slow cooling. No microstructural changes and no obvious difference in the Cu content in the 2:17 matrix phase were observed after slow cooling but the coercivity was significantly enhanced from 0.32 to 3 T. A large gradient of the Cu content in the cell boundary phase was detected in the highly coercive melt-spun Sm(Co,Fe,Cu,Zr)z ribbons with slow cooling by nanoprobe chemical analysis, in contrast to a homogeneous Cu distribution in the cell boundary phase of the ribbons without slow cooling. Further investigation revealed that a spinodal structure is developed in the Cu-rich Sm(Co,Cu)5 cell boundary phase of 2:17 SmCo magnets during slow cooling and the high coercivity of the 2:17 type magnets could result from the large gradient of domain wall energy within the cell boundary phase. © 2003 American Institute of Physics.