The effects of C additions on the phase transformation and magnetic properties of Sm(Co0.67-xFe0.25Cu0.06Zr0.02Cx)8.0, where x ranged from 0 to 0.15, melt spun ribbons and cast alloys have been studied by x-ray diffraction (XRD), differential thermal analysis (DTA), and vibrating sample magnetometer. In addition to the Th2Zn17 structure, two additional compounds, namely, the ZrC and SmCoC2, were detected by XRD after a thermal treatment over 700–1160 °C. The as-spun ribbons were highly crystalline at x=0 and became mostly amorphous at x=0.10. An intrinsic coercivity, Hci, of 3.0 kOe was obtained for the as-spun ribbons with x=0.05. After an optimum heat treatment, the Hci of the ribbons with x=0.01 was increased to 8 kOe. Cast alloys of identical chemical compositions were also solution treated and precipitation hardened. At x=0 for the cast alloy, a Br of 10.8 kG, Hci of 24 kOe, Hc of 9.8 kOe, and (BH)max of 27 MGOe, were obtained after an optimum heat treatment. © 1999 American Institute of Physics.