Farmers’ Participation in Voluntary Carbon Markets: An Integrated TPB–COM-B Analysis in Thailand

The voluntary carbon market (VCM) has emerged as a promising mechanism for climate mitigation; however, farmer participation in developing countries remains limited. This study combines the Theory of Planned Behavior (TPB) and the Capability–Opportunity–Motivation–Behavior (COM-B) framework to investigate factors associated with Thai farmers’ intention and self-reported stage of participation in VCM. Data were collected through face-to-face interviews with 240 farmers across multiple crop systems in Thailand and analyzed using Partial Least Squares Structural Equation Modeling (PLS-SEM). The model explained substantial variance in intention and behavior (R² = 0.610 and 0.555, respectively), although PLS-Predict indicated limited predictive performance. Perceived behavioral control (PBC) showed the strongest positive association with reported participation behavior (β = 0.493, p < 0.001), followed by intention (β = 0.343, p < 0.001). Access to extension and technical support (AES) was positively associated with intention (β = 0.624, p < 0.001) and PBC (β = 0.338, p < 0.001). Knowledge was positively associated with PBC (β = 0.324, p < 0.001) but negatively associated with intention (β = −0.106, p = 0.045). No significant association was observed between attitude and intention; however, subjective norms were negatively associated with intention (β = −0.336, p < 0.001). Indirect associations through intention and PBC were also observed. Overall, the findings suggest that capability-, opportunity-, and trust-related factors are associated with farmers’ reported participation in VCM and may inform the design of future policies and support programs. Although the model demonstrated useful explanatory capability, its predictive performance was limited, indicating that the findings should be interpreted primarily as explanatory rather than predictive.