Multiple-channel RF transmission holds great promise for MRI, especially for human applications at high fields. For calibration it requires mapping the effective RF magnetic fields, B(1) (+), of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B(1) (+) and tight SAR constraints. In the present work, this problem is revisited and solved by a novel mapping approach relying on an interference principle. The B(1) (+) fields of individual transmitter elements are measured indirectly by observing their interference with a SAR-efficient baseline RF field. In this fashion even small RF fields can be observed in the B(1) (+) -sensitive large-flip-angle regime. Based on a set of such experiments B(1) (+) maps of the individual transmitter channels are obtained by solving a linear inverse problem. Confounding relaxation and off-resonance effects are addressed by an extended signal model and nonlinear fitting. Using the novel approach, 2D mapping of an 8-channel transmitter array was accomplished in less than a minute. For validation it is demonstrated that mapping results do not vary with T(1) or parameters of the mapping sequence. In RF shimming experiments it is shown that the measured B(1) (+) maps accurately reflect the linearity of RF superposition.