This study examined various factors influencing the mucoadhesive properties of thiolated polymers (thiomers), which are capable of forming covalent bonds with thiol sub-structures of the mucus glycoprotein. Mediated by a carbodiimide, L-cysteine was therefore covalently bound to polycarbophil (PCP) and to carboxymethylcellulose (CMC). The resulting polymer conjugates displayed 12.3 and 22.3 micromol thiol groups per gram, respectively. Whereas the swelling behaviour of tablets based on CMC was not markedly influenced by the immobilisation of cysteine, it was improved significantly (P<0.05) in case of PCP. Tensile studies carried out with the unmodified and thiolated polymers of pH 3, 5 and 7, respectively, revealed that only if the polymer displays a pH-value of 5, the total work of adhesion can be improved significantly due to the covalent attachment of thiol groups. These results were in good agreement with a new mucoadhesion test system described here taking also the cohesiveness of the delivery system into account. The results represent helpful basic information in order to improve the mucoadhesive properties of thiolated polymers.