We discuss the use of the spectral range near 7570 Å to determine the Mg isotopic ratios in carbon stars using the B'2 Sigma +-X 2 Sigma + system of the MgH molecule. We also compare with the spectral range near 5140 Å that has been commonly used in normal stars (oxygen-rich stars). The region near 5140 Å is not the better choice for carbon stars because it is very crowded with CN and C_2 molecular bands and the localization of the continuum is complicated. The range near 7570 Å is less blended with molecular bands. The continuum is easy to locate and the isotopic splitting between MgH isotopic absorptions is larger. Unfortunately, we are not able to reproduce accurately the observed spectrum in this region and, moreover, the synthetic spectrum is not sensitive to large variations in the isotopic ratios.