Recycling and biodegradability of chain-growth polymers is an important and growing topic. Introducing ester bonds in polymers via cyclic ketene acetals (CKAs) is an interesting route to create (bio)degradability. Incorporation of CKA monomers is controlled by reactivity ratios. Reactivity ratios of different CKA/vinyl monomer systems published in the literature were re-evaluated with the nonlinear least-squares method, taking into account the error in the H-1 NMR measurements of monomer fractions in copolymers. This study confirms that the nonlinear least-squares method should be used instead of Fineman-Ross or Kellen-Tudos methods. Re-evaluated values suggest that reactivity ratios of CKA/vinyl monomer systems follow a family-like behavior.