The United States contains only a small proportion of the world’s tropical forests, but these are some of the most studied with regard to soil C pools, particularly forests in Hawaii and Puerto Rico. A comparison of soil C pools of these two subtropical islands provides a case study of the effects of substrate age, climate, and land-use history on tropical forest soil C. Hawaiian soils are derived from material that is ≤4 million years old, while Puerto Rican soils average approximately 40 million years old. To 25-cm depth, Hawaii had more soil C (91 ± 7 Mg/ha, n = 19) than Puerto Rico (75 ± 4 Mg/ha, n = 37). Mean annual temperature explained 45% of the variation in soil organic C (SOC) pools for the combined Hawaii and Puerto Rico data. Annual rainfall in wet and moist forests (>2500 mm per year) correlated linearly (r2 = 0.63, P < 0.01) with SOC pools in Puerto Rico. Soil age also plays an important role in the accumulation of SOC. The mineralogy of Hawaiian soils, and particularly the presence of amorphous minerals, results in greater long-term retention of SOC. In Puerto Rico, where highly weathered soils are unlikely to have much volcanically derived amorphous minerals, land-use history and forest age were useful indicators of 364soil C dynamics with time. In general, soils under secondary and plantation forests were sinks of SOC and appeared to be resilient to natural disturbance and human-induced events. Mature forests contained higher SOC pools at 0- to 25-cm depth than secondary forests recovering from land-use change. These two tropical islands are diverse in climate, mineralogy, human-disturbance regimes, and biodiversity, and they offer insights into the complex interactions that regulate SOC dynamics in forest ecosystems.