The magnetic and magnetocaloric properties of CoMnGe1 Ga-x(x) (0.02 <= x <= 0.063) alloys have been investigated by using calorimetric, X-ray diffraction and magnetic measurements. Our results show that the structural and magnetic phase transitions in CoMnGe1 Ga-x(x) alloys are tailored by the substitution of Ga for Ge. The CoMnGe1 Ga-x(x) (x = 0.05, 0.055 and 0.06) alloys exhibit a giant magnetocaloric effect due to coexistence of magnetic and Martensitic phase transitions at the same temperatures. The entropy change values range from -3.2 to -38.4 J kg (1) K (1) for Delta H = 7 T and are proportional to the volume differences of the phases involved in the transition. The adiabatic temperature change value of CoMnGe0.945Ga0.055 is calculated as 6.5 K for the magnetic field change of 2 T from magnetic and heat capacity measurements. The density of states of electrons at Fermi level and the Debye temperature values of this alloy are found 1.37 states eV (1) atom (1) and 337 K from low temperature heat capacity measurements. (C) 2013 Elsevier B. V. All rights reserved.