In liquid–solid adsorption, fluid film diffusion is typically faster than intraparticle diffusion, especially for microporous adsorbents. However, fluid film diffusion might play a significant role in the overall rate of the process for mesoporous–macroporous and non-porous solids. In most adsorption modeling studies, the fluid film diffusion step is typically ignored, which is not always justified. This article critically discusses the theory behind the liquid–solid mass-transfer coefficient in stirred vessels and presents the dissolution and adsorption methods adopted for estimating its value. Then, starting from the definition of the Biot number, an original analysis is developed with reference to selected studies. Surface versus pore diffusion of the adsorbate in the adsorbent is taken into account, and external versus internal mass-transfer resistance is considered to put the fluid film resistance back in the picture when needed. Iso-Biot charts where the operating points can be visualized are presented as well.