We report the demonstration of planar phased-array wavelength division multiplexers which incorporate two self- imaging multimode interference (MMI) couplers interconnected by an array of monomode waveguides. The MMI couplers operate as power splitters/combiners, and the waveguide array is the dispersive element. The excellent characteristics of MMI couplers offer the possibility of designing small-size devices with low loss and with high uniformity among different channels. Two variations of a 5-channel device with 2-nm channel spacing at 1550 nm have been designed and fabricated in a SiO2-SiON rib waveguide system. Both simulated and experimental performance of these multiplexers are presented. It is shown that the first device can function as a 5 X 5 wavelength selective interconnecting component. Also, it is demonstrated, through using a 1 X 5 MMI nonuniform power splitter in the second device, that sidelobes in the multiplexer spectral response can be suppressed by appropriately weighting the power samples in the array waveguides.