Compositions comprising carboxypolysaccharides (CPS) including carboxymethyl cellulose (CMC) and polyethylene glycols (PEGs) are provided where the PEG is a PEG-epoxide covalently linked to the CPS via an addition reaction. In certain embodiments, the PEG attaches to only one CPS, forming a decorated CPS. In other embodiments, bi-functional PEG molecules are attached to adjacent CPSs, thereby forming a covalently cross-linked composition. In certain of these embodiments, a PEG is linked to the CPS by way of an ether linkage, and in other embodiments, a PEG is linked to the CPS by way of an ester linkage, and in still further embodiments, PEG molecule(s) can be attached to CPS molecule(s) by way of both ether and ester linkages. Additional embodiments include PEG/CMC compositions where the PEG is a multi-branch PEG and/or a multi-arm PEG. PEG/CMC compositions can be made with desired viscoelastic properties, and such compositions can be used as space-filling materials, load-bearing materials, anti-adhesion compositions, for drug delivery vehicles or for lubrication of tissues and medical instruments.