In order to design devices based on II-VI materials, it is necessary to know the potential across the interface between two materials. Following our recent calculations which prove that the band gap energy of ZnSxSe1-y alloy has a nonlinear behaviour versus the sulphur composition x, it appears that an accurate knowledge of band offsets for ZnSxSe1-x/ZnSySe(1-y) structures will be useful to model devices based on this heterostructure. On the basis of a model-solid theory, we report in this work the band offset calculations for zinc blende pseudomorphically strained ZnSxSe1-x/ZnSySe(1-y) interface. From the results obtained, we have calculated the band gap energies of ZnSxSe1-x layers pseudomorphically strained on ZnSySe(1-y) substrate as a function of compositions x and y in the whole range 0 <= x,y <= 1. Also, the band gaps of bulk ZnSxSe1-x deposed on ZnSySe(1-y) for several values of y have been calculated versus the sulphur content x. Analytical formulas fitting these bands have been obtained. In view of the lack of theoretical calculations, our results seem likely to be useful especially in the design of ZnSxSe1-x structures for optoelectronic devices applications. (C) 2007 Elsevier B.V. All rights reserved.