This paper discusses an investigation of the effects of mirror quality on the performance of reflecting arrayed-waveguide-grating multiplexers. Using specially designed external mirrors as external reflectors, the optimum flatness and microroughness of the reflecting surface are determined. The experiments discussed in the paper indicate that a reflecting-surface flatness of λ/4 is sufficient to obtain high-performance reflective multiplexers. Extrinsic mirror-related phase and intensity errors were found to be negligible for surfaces polished with diamond lapping films with a grain size smaller than 1 μm. These experimental results were found to be in excellent agreement with simulations based on mirror-related changes in the waveguide-grating-length increment.