Optical characteristics of the top and bottom cells in Cd1-xMgxTe/CuInSe2 tandem solar cells with transparent ZnTe interconnection layer are investigated. Since the reflection coefficient is significantly attenuated in the case of Cd1-xMgxTe/ZnTe interface in comparison to the Cd1-xMgxTe/glass interface, the transmission through the monolithic top cell of the tandem can be calculated ignoring the oscillations. It has been shown that the short-circuit currents in the top and bottom solar cells match when the bandgap of Cd1-xMgxTe is in the range 1.67-1.70 eV. A good agreement between the measured and calculated periodic oscillations in the top cell transmittance has been demonstrated. It has been shown that the current matching can be achieved with a lower band gap Cd1-xMgxTe top-cell when borosilicate glass substrate is used, and also the short circuit current density is higher, compared to soda lime glass. (c) 2015 Elsevier Ltd. All rights reserved.