Telechelic anionic and cationic biodegradable ionomers capable of loading, storing, and releasing proteins are presented. Two different ionomers have been synthesized with either anionic or cationic end groups. The reaction was done quantitatively as shown by H-1 NMR. The swelling properties of the hydrophobic poly( trimethylene carbonate) polymer are contributed to the ionic end groups that display hydrophilic properties. Depending on the molecular weight of the ionomer, and also on the ionic charge, the materials swell differently in water, from similar to 50% for M-w = 12 000 g/mol to similar to 500% when dealing with 2000 g/mol. The high swelling led us to believe that it would be possible to load and release proteins preferably in a still active form. As models, two different proteins were chosen: hemoglobin and cytochrome c. The swelling and release study shows that both ionomers possess the capability to adsorb and later release the proteins with retained structure. Release measurements from both the swollen and dried states have been evaluated with similar results, showing that the dried state seems to release a little bit less than the swollen one. These kinds of materials should be interesting for a wide variety of applications where drug and protein release is wanted, as well as in applications such as protein separation media.