The effect of substituting Sr for Ba on the magneto-transport and magnetic properties of the (La2/3Sm1/3)0.67Ba 0.33MnO3 system has been investigated. The samples of (La2/3Sm1/3)0.67Ba0.33-xSr xMnO3 (x ≤ 0.0, 0.1, 0.2 and 0.33) are synthesized by the citrate gel route and crystallize in an orthorhombic structure. The unit cell volume decreases while the metal-insulator transition temperature (T MI) increases with increasing Sr content. The atomic disorder, which is represented by the size variance, increases with Ba content. The small anomaly in ρ(T) due to the atomic disorder which leads to the different magnetic ordering followed by a broad metal-insulator transition is observed in Ba-rich systems. The peak at T* (small anomaly) is presumed to be due to spin-dependent interfacial tunnelling that causes the difference in magnetic order between the surface and the core. The suppression in the resistivity values with increasing Sr content could be due to the more symmetric MnO6 octahedra network. The paramagnetic phase coexists with the magnetically ordered phase between T* (∼TC) and TMI. The suppression of the metastable magnetic phase leads to the high magnetoresistance in Ba-rich samples. The transport data has been analysed using a small polaron correlated model. The magnetic behaviour may be understood by the Ising model. The overall magnetic behaviour could be accounted for by the competing antiferromagnetic and ferromagnetic interactions. © 2007 IOP Publishing Ltd.