Acceleration of thermal donor formation at 400 °C in Czochralski Si by a hydrogen plasma has been observed using low‐temperature infrared absorption and spreading resistance probe measurements. The accelerated formation in as‐grown Si is attributed to hydrogen diffusion and catalyzed conversion of electrically inactive nuclei to thermal donors. When the nuclei concentration is small such as in rapid thermal annealed Si, hydrogen interaction at SiO bonds is suggested as the rate‐limiting step in thermal donor formation.