The uptake of tridodecylmethylammonium chloride (TDMAC) by poly(vinyl chloride) has been investigated to provide a more quantitative basis for the preparation of blood-compatible surfaces based on TDMAC-heparin coatings. Sorption isotherms of TDMAC from toluene-cyclohexane and toluene-methanol mixtures have been measured. In toluene-cyclohexane mixtures, the TDMAC uptake is proportional to the degree of swelling of the polymer. From ion-exchange experiments with 36Cl-, it appears that only a small fraction of the TDMAC remains near the PVC surface to provide the heparin binding capacity. Methanol forms a strong H-bonded complex with TDMAC in toluence and prevents its sorption by PVC.