The effect of wall tilt on the performance of directional couplers has been studied. There is a significant reduction in the coupling length as the tilt decreases when nonvertical walls, which are inherent in nearly all clean-room waveguide fabrication processes, are considered. Comparison of 90 degrees and 45 degrees coupling lengths reveals a difference of nearly a factor of 2 between them. Experimental data obtained with antiresonant reflecting optical waveguide directional couplers confirm that tilt effects should be considered in any device with large core thickness.