To determine how the conversion of mature tropical forests to secondary forests affects the soil C budget, major soil C storages, input and CO2 evolution from a tropical inceptisol were measured over a 6-mo. period in both a mature lowland Tropical Premontane Wet Forest and a nearby secondary site located on the same soil type. Total C storage in and on the mature forest soil was composed of 9330 gc/m2 in soil organic matter, 1850 gC/m2 in litter and 340 gC/m2 in small roots (diameters < 5 mm); larger roots were not measured. Average daily inputs to the mature forest soil included 1.3 gC/m2 in litterfall and 0.10 gDOC[dissolved organic C]/m2 in precipitation (throughfall + stem flow). The evolution of CO2 from the mature forest soil averaged 3.4 gC/m2 .cntdot. d [day] or 2.6 times the average rate of litterfall. Total C storage in and on the soil of the 2nd-growth was composed of 8600 gC/m2 in soil organic matter, 700 gC/m2 in litter and 157 gC/m2 in small roots, or 2060 gC/m2 less than in the mature forest. Litterfall in the young 2nd-growth averaged 0.7 gC/m2 .cntdot. d, and precipitation averaged 0.12 gDOC/m2 .cntdot. d. Soil-CO2 evolution averaged 4.6 gC/m2 .cntdot. d, or 1.4 times the rate in the mature forest. Measured inputs of C to the soil were considerably less than soil-CO2 evolution rates in both sites; part or all of these differences can be attributed to root production, which was not measured. Forest conversion to young secondary vegetation resulted in losses of soil organic matter, fewer small roots, less non-woody and large wood litter, lower rates of litterfall, and increased rates of soil-CO2 evolution.