To detect the effects of a rye cover crop on labile soil carbon, the light fraction, large  particulate organic matter (POM), small POM, and two NaOH-extractable humic fractions were extracted from three depths of a corn soil in central Iowa having an overwinter rye cover crop treatment and a control lacking rye cover crop.  In the surface 5-cm layer, the light fraction and both POM fractions had substantially larger masses with the rye cover crop than in the control at a mid-June sampling but differences were less by a second sampling in early September.  Smaller or no mass differences were found in the 5-15 and 15-30 cm depths. In contrast, the younger of the two humic fractions had larger masses in the control than with the rye cover crop in all three depths in June, and differences were muted by September.  In the 0-5 cm layer, the rye cover crop tended to promote an enrichment of (1) the plant carbohydrate xylose in all fractions in June, (2) several other carbohydrates in both humic fractions and whole soil in both June and September, and (3) phenols in both humic fractions in September.  The rye cover crop also tended to promote enrichment of amino acids in whole soil, but trends in fractions were less clear.   In this study, the rye cover crop affected sizes and chemical natures of labile carbon pools, even if total soil carbon was not clearly affected.