The active fraction of soil organic carbon (SOC) is an important component of soil health and often is quickly assessed as the amount of CO<sub>2</sub> released by re-wetting dried soils in short-term (24–72 h) assays. However, soils can lose carbon (C) as they dry and if soil samples vary in moisture content at sampling, differential C loss during the pre-assay dry-down period may complicate interpretations of C availability. We examined pre-assay CO<sub>2</sub> loss and its influence on apparent C availability in the same soil at initial moisture contents of 30, 50, and 70 % water-filled pore space (WFPS). We found that 50 and 70 % WFPS treatments lost more C during drying than those in the 30 % WFPS treatment, which led to a 26–32 % underestimate of C availability in wetter soil. We developed a soil-specific correction factor to account for these initial soil moisture effects. Future C mineralization studies may benefit from similar corrections.