Fouling in heat exchangers is the unwanted buildup of material on the heat exchange surface which reduces thermal efficiency. The crude preheat train usually consists of a set of heat exchangers which tend to become fouled. Crude oil fouling can be described by the following steps: formation of particles (either by chemical reaction, corrosion or asphaltene precipitation); transport to the surface; adsorption of fluid constituents; desorption of the fouling product; transport of the fouling product to the bulk fluid. Assuming the single steps are rate controlling, fouling rate equations were developed which show the influence of the main variables fluid velocity and temperature on the fouling rate. All steps were assumed to be in series. The developed equations were used to determine the controlling step of fouling by comparing experimental data. The results show that no matter what the fouling mechanism was, the effects of velocity and temperature on fouling rates could best be described with the developed equations for fouling controlled by adsorption. This leads to the assumption that either single steps or a binary combination of steps affect the fouling process, with adsorption always being a decisive step.