The effect of substitutional C on interstitial‐enhanced B diffusion in Si has been investigated. Substitutional C was incorporated into B doped Si superlattices using molecular‐beam‐epitaxial growth under a background of acetylene gas. Excess Si self‐interstitials were generated by near‐surface 5×1013/cm2, 40 keV Si implants and diffused at 790 °C. The interstitial‐enhanced diffusion of the B marker layers is fully suppressed for C concentrations of 2×1019/cm3, thus demonstrating that substitutional C acts as a trap for interstitials in crystalline Si. Uniform C incorporation of 5×1018/cm2 significantly reduces the transient enhanced diffusion of a typical B junction implant without perturbing its electrical activity.