The purpose of this work was to show that the overall peak power of RF pulses for CAIPIRINHA excitation can be substantially reduced by applying interslice phase relaxation. The optimal phases are scan dependent and can be quickly calculated by the proposed method. The multi-band RF pulse design is implemented as the minimization of a linear objective function with quadratic constraints. The interslice phase is considered to be a variable for optimization. In the case of a phase cycling scheme (CAIPIRINHA), the peak power is considered over all pulses. The computation time (about 1s) is compatible with online RF pulse design. It is shown that the optimal interslice phases depend on the CAIPIRINHA scheme used and that RF peak power is reduced when the CAIPIRINHA phase cycling is taken into account in the optimization. The proposed method is extremely fast and results in RF pulses with low peak power for CAIPIRINHA excitation. The MATLAB implementation is given in the appendix; it allows for online determination of scan-dependent phase parameters. Furthermore, the method can be easily extended to pTx shimming systems in the context of multi-slice excitations, and this possibility is included in the software. Copyright (c) 2015 John Wiley & Sons, Ltd.