A series of molecular dynamics simulations were performed to investigate the effects of model asphaltenes on the toluene/water interfacial tension (IFT) under high temperature and pressure conditions. In the absence of model asphaltenes, the toluene/water IFT monotonically decreases with increasing temperature, whereas, with the presence of model asphaltenes, especially at high concentrations, such monotonic dependence no longer holds. Furthermore, in contrast with the decreasing trend of IFT with increasing model asphaltene concentration at low temperature (300 K), increasing concentration at high temperature (473 K) leads to increasing IFT. This relation can even be nonmonotonic at moderate temperatures (373 and 423 K). Through detailed analysis on the distribution of model asphaltenes with respect to the interface, such complex behaviors are found to result from the delicate balance between miscibility of toluene/water phases, solubility of model asphaltenes, and hydrogen bonds formed between water and model asphaltenes. By increasing the temperature, the solubility of model asphaltenes in toluene is enhanced, leading to their transition from being a surfactant to being a solute. The effect of pressure was found to be very limited under all model asphaltene concentrations. Our results here present, for the first time, a complete picture of the coupled effect of (high) temperature and asphaltene concentration on IFT, and the methodology employed can be extended to many other two-phase or multiphase systems in the presence of interface-active chemicals.