The influence of small deviations of Mn from stoichiometric composition on the crystal structure and magnetic properties of Ni 2 Mn 0.75(1−x) Cu 0.25 Ga Heusler alloys has been studied using X-ray diffraction, thermal expansion, and magnetization measurements. The samples were found to be of mixed phases containing cubic (austenitic) and tetragonal (martensitic) structures at room temperature. The temperatures of the martensitic transformations and ferromagnetic ordering of the austenitic phase coincide at T=T MS for all the samples, and vary from 310 to 333K as the Mn deficiency (x) changes from 0 to 0.06. The magnetic entropy change (ΔS M ) in the vicinity of T MS remains nearly constant for the entire Mn concentration interval with a value of about −45Jkg −1 K −1 for a magnetic field change of 5T. The refrigeration capacity was found to change from 69Jkg −1 (for x=0) to 88Jkg −1 (for x=0.01) to 40Jkg −1 (for x=0.06).