Polysarcosine (pSar) was one of the first polymers synthesized in a controlled living manner, but it was only recently when it was reconsidered as a promising alternative for poly(ethylene glycol) (PEG) in biomedical applications. Despite receiving more and more attention, very little is known about the solution properties of pSar, such as coil dimensions and thermodynamic interactions. In this article, we report on these properties of pSar with degrees of polymerization 50 < X-n < 400 that were prepared by controlled living ring-opening polymerization. The polymers are characterized by gel permeation chromatography (GPC), MALDI-TOF mass spectrometry, dynamic and static light scattering (SLS), and viscometry. The chain stiffness of pSar in PBS in terms of the Kuhn statistical segment length, l(k), was estimated to l(k) = 1.5 nm by application of the Yamakawa-Fujii wormlike chain theory to the experimentally determined hydrodynamic radii, R-h, thus being higher than l(k) = 1.1 nm for PEG in PBS. Also, the second virial coefficients, A(2), of pSar and PEG in PBS were similar and reflect their good solubility in aqueous solution. While the universal calibration of GPC elution volumes failed for pSar in HFIP utilizing PMMA standards, it worked better in PBS buffer with PEG standards. Alternatively, an R-h-M-w relation is established in the present work, which enables the determination of molar masses of pSar by simple DLS measurements. In addition, it is demonstrated that pSar independent from its chain length (50 < X-n < 400) does not induce any detectable complement activation (C5a) in human serum.