We discuss the design of Mach-Zehnder wavelength duplexers for 1300 nm and 1530 nm band-centres using InGaAsP/InP strip-loaded waveguides. We show that this device has a high inherent tolerance to fabrication and waveguide parameter variation and is therefore particularly suitable for ‘dead-reckoned’ fabrication in a low-cost optoelectronic transceiver chip for local area fibre networks. Using a novel configuration of concentrically curved waveguides, crosstalk better than —11.75 dB has been achieved in both channels. The performance of the constituent waveguide bends and directional couplers is also discussed.