We have intermixed GaAs/(AlGa)As multiquantum structures for waveguides and lasers by 3 MeV Ar implantation and 850 °C, 30 min closed‐tube annealing. Buried‐heterostructure lasers defined by Ar mixing had threshold currents of 100 mA for 370‐&mgr;m‐long devices. As waveguides for 1.15 &mgr;m light, the devices exhibited losses of 25 cm−1in the annealed, implanted regions, and 15 cm−1in unimplanted regions defined by adjacent implants. Analysis of the results illustrates important considerations for implant mixing for waveguide formation.