Multijunction solar cells are exposed to reverse bias operation under certain situations during their life. Even though the cells are protected with bypass diodes, the limiting subcell within the multijunction stack can be in reverse operation in some cases. This work assesses the reverse behaviour of component and triple junction cells using a model based on the Ecosim environment. Dynamic effects have been considered in the modelling of the parallel resistance, in order to get a more accurate simulation of the performance in reverse. The simulation tool has been expanded to four-junction devices, with good preliminary results under different spectral conditions.