A novel unsaturated-flow pressurized reactor (UFPR) for hydrogenotrophic denitrification was recently developed. The reactor is characterized by safe and economic operation since gas purging intrinsic to conventional H-2-based systems is not required and H-2 loss is limited only to the dissolved H-2 in the effluent. Additionally, high denitrification rates are achieved by high water recirculation over plastic carriers with high surface area. This paper focuses on mathematical modeling of the novel reactor, based on its unique and specific characteristics. The continuously stirred hydraulic regime formed due to the relatively high recirculation flow rate required for efficient media wetting and the homogeneous gas phase in the closed reactor head space, simplified the model design for the UFPR. The reaction rate constant and the overall volumetric gas (H-2)-liquid mass transfer coefficient (k(L)a) were determined for different recirculation flow rates at steady state. A rate constant correction factor beta was developed to compensate for pH changes within the biofilm, deviation from intrinsic zero-order degradation kinetics and non-homogeneity of the biofilm. Model validation tests showed a high correlation between experimental and model results for various combinations of operational parameters. Results from the model showed that high denitrification rates of up to 7.5 g NO3--N/(L-reactor.d) together with H-2 utilization efficiencies above 90% can be achieved by the UFPR. (C) 2016 Elsevier B.V. All rights reserved.