In this study, the effect of the coat thickness of polyacrylate-polymethacrylate copolymer on the adsorption capacity of activated charcoal to methylene blue as a model marker was investigated by constructing both Langmuir and Freundlich isotherms. It was found that the coat thickness significantly affected the adsorption coefficient of the coated charcoal to methylene blue as is reflected by the attractive forces between the adsorbent and the adsorbate. This effect is understandable as the membrane trill act as a barrier between the adsorbent and the adsorbate. The coat thickness also had some effect on the adsorption capacity of activated charcoal but not as much as it affected the adsorption coefficient. The data followed the Freundlich isotherm more closely than the Langmuir equation. The effect of the coat thickness on the adsorption of selected drugs of different molecular size was also investigated using theophylline, paracetamol, sodium phenobarbital, creatinine and vitamin B-12 as model drugs. The results showed that the adsorption patterns of theophylline, paracetamol and sodium phenobarbital were more or less similar. The apparent coating thickness did not affect the extent of adsorption of these model drugs and there was little effect on the adsorption rate especially during the first 15 min. The adsorption of vitamin B(1)2 and creatinine showed completely different patterns which are discussed in detail.