Poly(ether-ester)s composed of hydrophilic poly(ethylene glycol)-terephthalate (PEGT) blocks and hydrophobic poly(butylene terephthalate) (PBT) blocks were studied as matrix for the controlled release of calcitonin. Salmon calcitonin loaded PEGT/PBT films were prepared from water-in-oil emulsions. The initial calcitonin release rate could be tailored by the copolymer composition, but incomplete release of calcitonin was observed. FTIR measurements indicated aggregation of calcitonin in the matrix, which was not due to the preparation method of the matrices, but due to the instability of calcitonin in an aqueous environment. Release experiments showed the susceptibility of calcitonin towards the composition of the release medium, in particular to the presence of metal ions. With increasing amount of sodium ions, a decrease in the total amount of released calcitonin was observed due to enhanced aggregation. The calcitonin had to be stabilized in the matrix to prevent aggregation. Incorporation of sodium dodecyl sulphate (SDS) as a stabilizer in PEGT/PBT matrices increased the percentage of calcitonin released, but could not avoid aggregation on a longer term.