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We have synthesized a series of Co-doped Mn0.5Fe0.5Ni1−xCoxSi0.94Al0.06 (0.025 ≤ x ≤ 0.05) alloys by arc melting followed by a rapidly quenched vacuum suction casting technique and studied the magnetic and magnetocaloric properties of the system. All measurements were performed on the as-cast and annealed samples. X-ray diffraction data indicated that the samples exhibited a hexagonal phase at room temperature. Magnetization data showed that the annealed samples exhibited significantly larger transition temperatures than the as-cast samples. All samples showed the first-order phase transition with a thermomagnetic hysteresis of more than 20 K. The phase transitions were accompanied by isothermal entropy changes of as large as −16.4 J kg−1 K−1 and −42.4 J kg−1 K−1 for field changes of 20 and 50 kOe, respectively. Large refrigeration capacities and temperature averaged entropy changes of up to 201 J/kg and 34.05 J kg−1 K−1, respectively, were observed for a field change of 50 kOe.