To understand the sensitivity of soil organic carbon (SOC) in peat soils planted with sago palm to increases in pH, temperature, and solar irradiation, four soil samples collected from two sago palm gardens (SGs) in Mukah, SG1 and SG2, and a SG and a neighboring forest in Talau, Sarawak, Malaysia, were subjected to laboratory incubation experiments. ¹³C nuclear magnetic resonance spectra were obtained to characterize the SOC of each soil. Three soil sample treatments: control (native pH (3.3‒4.2), 25°C), neutral pH (pH 7, 25°C), and higher temperature (native pH, 35°C), were incubated for 91 days under aerobic conditions. The rate of production of carbon dioxide was determined at intervals of 7‒14 days. A photodegradation experiment was conducted using Talau soil samples at 650 W m^-2 for seven days. Two Mukah SG soils were characterized by an enrichment of alkyl C. In the Talau SG soil, both O-alkyl C and alkyl C were dominant, while aromatic C, followed by O-alkyl C, was dominant in the Talau forest soil. The rate of SOC mineralization in the control treatment was higher in the order of Mukah SG1, Mukah SG2, and two Talau soils, suggesting an enhancement of the decomposability of SOC under sago palm cultivation in association with the transition in C composition. The total amount of SOC mineralized in the neutral pH treatment in three of four soils was 1.4‒3.2 times larger than that in the control treatment, although the amount may still be underestimated because CO₂ dissolved in soil water has not been evaluated. Higher temperature treatment also enlarged the amount of SOC mineralized to 1.6‒3.1 times that of the control treatment in all soils. After seven days of irradiation, SOC decreased by 4‒16% of the initial, suggesting that photodegradation is a possible mechanism of significant C loss in shallow surface soil.