Abstract The compounds MgCo2−2xZn2x form a series of solid solution over the whole concentration range. The crystal structures correspond to hexagonal (C14) when x ⪕ 0.2 and x ⪖ 0.63. In the intermediate range the structure is cubic (C15). Replacement of Co in MgCo2 by Fe is only possible up to 10% Fe. In the first series increasing Zn concentration leads to a gradual reduction of the magnetization and the Curie temperature. In MgCo1.8Fe0.2 the magnetization is similar to that in MgCo2 but the Curie temperature is higher. The substitution of Zn and also of Fe for Co leads to a disappearance of the anomalous reduction of the low temperature magnetization observed in MgCo2. 59Co Spin Echo NMR has been used to determine the magnitude and sign of the hyperfine fields at the two Co sites in MgCo2 and to study the temperature dependence of these Co hyperfine fields. No discontinuity is found in the hyperfine field near the temperature where the bulk magnetization rises steeply. This excludes a possible explanation of the anomaly in terms of changes of the intrinsic Co moments. Furthermore, the intensity of the spin echo signals (NMR enhancement factor η ≈ 70) remains roughly the same within wide limits of the temperature where the anomaly is observed. This refutes an explanation in terms of a ferromagnetic-to-antiferromagnetic transition, proposed earlier. The appearance of a broadened spin echo spectrum (NMR enhancement factor η ≈ 7000) at low temperatures, in addition to the relatively sharp resonances observed also at higher temperatures, has been interpreted as being due to easy nucleation of domain walls at defect sites with a lower magnetic coupling.