We have investigated the ring-opening polymerization (ROP) of ε-caprolactone using mixtures of methanesulfonic acid and the guanidine 1,5,7-triazabicyclo[4.4.0]dec-5-ene as the catalyst. Our interest in these mixtures is based on the capability of both acids and bases to behave as bifunctional catalysts; the former by the combined action of acidic hydrogens and basic oxygens, and the latter by the hydrogen-bond acceptor and donor features of, respectively, a basic nitrogen center and a ortho-hydrogen atom. We found that these compounds formed either eutectic (DES) or non-eutectic mixtures depending on the molar ratio in which the two components were mixed. Upon the use of these mixtures as catalysts, neither further solvents nor initiators were required to carry out the ROP of ε-caprolactone. The resulting PCLs were highly crystalline (more than 87%) and exhibited an excellent capability to support the growth of murine L929 fibroblasts. We consider that the preparation of biocompatible PCLs at physiological temperatures and in the absence of further reagents than the monomer and the catalyst offers an interesting alternative to both the self-cross-linked oligomers/macromers of acrylate-based PCL-derivatives and the pH/temperature-sensitive PCL copolymers used to date as injectable biomaterials.