The instabilities occurring in forced-convection upflow passing through a parallel-channel system have been studied theoretically and experimentally, using water as test fluid. A non-linear mathematical model is proposed, that explains the results quite satisfactorily. Close agreement is obtained between calculations based on this model and measure-ments of the stable boundary in a system of two channels. The model is further applied to three and four parallel channels, the results of which indicate that the flow instability behavior in such multiple channels are quite similar to that observed in the two-channel system, in such aspects as the stable boundary, as well as the period and the development of flow oscillations in the unstable region.