Most rumen mechanistic models adopt a zero pool for hydrogen and estimate methane production based on hydrogen sources and sinks. A dynamic mechanistic model that represented substrate degradation, volatile fatty acid (VFA) production pathways, and methanogenesis in the bovine rumen was developed. This preliminary model also represented the thermodynamic control of H₂ partial pressure (pH₂) on the type of VFA formed via the NAD⁺ to NADH ratio (r_NAD). Feed composition and intake rate (twice-daily feeding regimen) observations were used as model input. Model parameters were estimated to experimental data using a Bayesian calibration procedure, after which the uncertainty of the parameter distribution on the model output was assessed. This Bayesian mechanistic modeling effort is unique in providing a mathematical representation of diurnal dynamics of VFA, H₂ and CH₄ production in the bovine rumen, in which the type of VFA is controlled by pH₂ via r_NAD homeostasis, based on principles of reaction kinetics and thermodynamics. The preliminary model predicted a marked peak in pH₂ after feeding that rapidly declined in time. This peak in pH₂ caused a decrease in r_NAD followed by an increased propionate molar proportion at the expense of acetate molar proportion. In response to feeding, the model predicted an increase in CH₄ production that steadily decreased in time. The pattern of CH₄ emission rate followed the patterns of pH₂ and H₂ emission rate, but its magnitude of increase in response to feeding was less pronounced. A global sensitivity analysis was performed to determine the impact of parameters on daily CH₄ production. The parameter that determines the NADH oxidation rate explained 41% of the variation in predicted daily CH₄ emission. The preliminary model was evaluated using 40 measurements from 3 experiments conducted at Wageningen University. Model evaluation indicated daily CH₄ production to be under-predicted, and showed a root mean square prediction error of 15%. The present modeling effort provides the integration of more detailed knowledge than in previous rumen fermentation models and allows assessment of diurnal dynamics of rumen metabolic pathways yielding VFA, H₂ and CH₄.