The structural and magnetic properties of Ni(50)Mn(25+x)Sb(25-x) (0≤x≤18) Heusler alloys have been investigated by x-ray diffraction, magnetization, thermal expansion, and electrical resistivity measurements. Austenitic phases with the L2(1) cubic crystal cell and martensitic phases with Pmm2 orthorhombic structures have been observed at room temperature in alloys with 0≤x≤12.5 and 13≤x≤14, respectively. The Curie temperatures of the austenitic phases decrease linearly with increasing x and change from 370 (x = 0) to 340 K (x = 12.5). In the concentration 7<x≤10, the system is found to undergo martensitic phase transitions above 150 K. The martensitic transition temperatures increase rapidly with increasing x and for x≥13.5 the temperature exceeds the Curie temperature of the austenitic phase (340 K). The alloys in their martensitic phases exhibit complex magnetic behavior. The phase diagram with respect to temperature and composition of the Ni(50)Mn(25+x)Sb(25-x) system has been determined, where the following magnetic phases have been observed in the martensitic state: antiferromagnetic phase (14<x); paramagnetic and antiferromagnetic (13≤x≤14, above 120 K) mixed phase; ferromagnetic and antiferromagnetic (10≤x≤14, above 20 K) mixed phase. The mixed phases result in observation of the exchange bias effect at low temperatures (below 120 K).