A mechanistic model was used to describe how mass and energy exchange between a leaf and air affect internal pressurization in aquatic plants. The core of the model is the calculation of the temperature of a leaf for which the energy balance is zero. The leaf temperature and water vapour gradient between leaf and air are then used to calculate potential pressurization. Simulations with the model were used to demonstrate the sensitivity of gas pressurization in aquatic plants to key environmental and physiological factors including radiation, temperature, humidity, wind speed, leaf size and leaf conductance. The model confirms other published data showing that humidity-induced pressurization is the dominant mode of pressurization under most conditions. The simulations also demonstrated that thermal transpiration and humidity-induced pressurization are not separate phenomena, but are dependent on the energy balance of the leaf, which in turn is affected by complex interactions between the leaf and environment.