Ni2MnGa that possesses a Heusler L21 structure undergoes a martensitic transformation from the parent cubic (austenitic) phase to a low temperature complex tetragonal structure at TM = 202 K and has a Curie temperature of TC = 376 K. Some research groups have observed an intermartensitic transformation at a temperature TI<TM, in Ni2MnGa single crystals. In this work detailed studies on the influence of substitution in the Mn subsystem by magnetic Co on the intermartensitic transformation properties of Ni2Mn1−xCoxGa compounds have been done by magnetization (5–400 K) and thermal expansion (80–300 K) measurements. The samples Ni2Mn0.92Co.08Ga, Ni2Mn0.91Co.09Ga, and Ni2Mn0.90Co.10Ga were studied. The intermartensitic transformation is observed in all of these alloys. As the Co concentration increases, the transformation becomes more pronounced and the temperature range for which the alloys stay in the intermartensitic state decreases as Co concentration increases. Similarities were observed between the magnetization and thermal expansion curves. The results are discussed in terms of the internal stress produced as a result of the Co substitution.