We report on the electrical and structural characterization of sulfur (S) doped Ga0.73In0.27As layers, grown on SI (001) GaAs substrates by chemical beam epitaxy. The room temperature free electron concentration is 2×1017cm−3 while the corresponding value of mobility is 3400 cm2V−1 s−1. The epilayer is characterized by a deep trap, which could be attributed to the electrical activity of dislocations, with an activation energy of 0.59 eV and a capture cross section 6×10−15 cm2. TEM analysis shows that the GaInAs/GaAs interface is characterized by dislocation lines and loop-like configurations which could be attributed to climb movement by point defect absorption or emission. Annealing at 420°C (Ar ambient for 5 min) does not alter the carrier concentration (n) and mobility (μ) significantly. The invariance of n and μ, even though the temperature should be high enough to dissociate any Satomic hydrogen complexes, indicates that the number of hydrogen-S donor complexes in the as-grown material is small compared with the donor concentration. Finally, the effect of intentional atomic hydrogen diffusion is discussed.