[en] The hyperfine magnetic fields acting on ¹⁸¹ Ta nuclei at the Sc and Hf sites have been measured in Heusler alloys Co₂ Sc Sn and Co₂ Sc Ga and Co₂ Hf Sn using the Time Differential Perturbed γ-γ Angular Correlation (TDPAC) technique. The measurements were carried out using an automatic spectrometer consisting of two Ba F₂ detectors and the conventional electronics. The magnitude of hyperfine magnetic field at ¹⁸¹ Ta was measured for all the alloys. The signs of the were determined in the cases of Co₂ Sc Sn and Co₂ Hf Sn alloys by performing the Perturbed Angular Correlation measurements with an external polarizing magnetic field of ≅ 5 k Gauss. The hyperfine magnetic fields obtained are -187,6± 3,3 and 90,0 ± 2,1 kOe measured at 77 K for Co₂ Sc Sn and Co₂ Sc Ga alloys respectively, and -342,4 ± 10,1 kOe measured at the room temperature for Co₂ Hf Sn alloy. These results are discussed and compared with the hyperfine magnetic field systematics in Co-based Heusler alloy. (author)