Frequently applied micro models for gas-liquid mass transfer all assume the presence of a liquid bulk. However, some systems are characterized by the absence of a liquid bulk, a very thin layer of liquid flows over a solid surface. An example of such a process is absorption in a column equipped with structured packing elements. The penetration model was slightly modified, so I that nit can describe systems without liquid bulk. A comparison is made between the results obtained with the modified model and the results that would be obtained when applying the original penetration theory for systems with liquid bulk. Both physical absorption and absorption accompanied by first and second order chemical reaction have been investigated. It is concluded that the original penetration theory can be applied for systems without liquid bulk, provided that the liquid layer has sufficient thickness (delta > d(pen)(*)). For packed columns this means, in terms of Sherwood number, Sh >= 4. In case of a 1,1-reaction with Ha > 0.2 an additional second criterion is Sh >= 4 root/D-b/D-a For very thin liquid layers (Sh < 4 or Sh < 4 root D-b/D-a), the origional penetration model may give erroneous results, depending on the exact physical and chemical parameters, and the modified model is required.