It is shown for the first time that carbon behaves predominantly as an acceptor in InGaAs and AlInAs under co‐implantation conditions. The co‐implanted ion, regardless of species, acts to create vacant lattice sites for occupation by the carbon. Implantation of 40 keV carbon ions alone at doses between 5×1012and 5×1014cm−2followed by annealing in the range 600–950 °C for 10 s does not produce any measurable electrical activity in either material. In InGaAs, carbon implantation at 5×1014cm−2produced net acceptor activations of 20, 11, or 6% for Ga, Ar, or As co‐implantation, respectively, at the same doses after 700 °C, 10 s anneals. Similar results were obtained for AlInAs after annealing at 900 °C. The diffusion coefficient for carbon is estimated from secondary‐ion mass spectrometry measurements to be less than 3.3×10−14cm2  s−1at 800 °C in both materials.