demonstrated that solutions poly(2-iso-propyl-2-oxazoline) (Mw: 2.4 kg/mol, 1 g/L) in D2O had a Tcp 2.5 °C lower compared to H2O . Here, we conducted turbidity measurements in D2O for S12 and S0 at 5 g/L (Figure ) and found consistently decreased Tcp in D2O compared to H2O. On average, Tcp's decreased by 1.2 (±0.7) °C for S12 and 1.7 (±0.8) °C for S0, respectively. Another polymer with tertiary amide repeating units, poly(N-vinylcaprolactam), also exhibits a 0.6 °C lower Tcp in D2O. Therefore, it appears that for polymers with tertiary amides, the intensified hydrophobic effect dominates over the stronger polymer-water H-bonding, in sum reducing the Tcp. In contrast, for poly(N-iso-propylacrylamide) (PNIPAM), the inverse behavior is observed with 0.6 to 1 °C higher Tcp in D2O. This suggest that for polymers with secondary amides the stronger hydrogen bonding in D2O for PNIPAM dominates over the increased hydrophobic effect, effectively increasing the Tcp. The phase transition of PNIPAM upon heating starts from backbone aggregation; therefore, side chain interactions with better hydrogen bonding in D2O results in slightly better solubilization in D2O. Similarly, improved hydrogen bonding in D2O can be observed for the proteins with peptide bonds containing secondary amide repeating units. They exhibit enhanced thermal stability and increased denaturation temperatures in D2O. Our results of the turbidity measurements in D2O suggest that the effect of stronger hydrophobic interactions in D2O dominates over the effect of stronger D2O-polymer hydrogen bonding, which results in a decreased Tcp for each sample. However further investigation is needed for a better understanding of the phase transition of thionated polymers. This should include conducting similar experiments with thionated polymers bearing secondary amides, for example PNIPAM.