Crystalline naloxol-peg conjugate

Provided are naloxol-polyethlyene glycol conjugates in oxalate or phosphate salt forms including crystalline forms. Further provided are methods of preparing the salt forms and pharmaceutical compositions comprising the salt forms. The methods for preparing the oxalate salt comprise dissolving the naloxol-polyethylene conjugate free base in 2 relative volumes of acetonitrile or ethanol, adding 3 equivalents of water or 8 relative volumes of methyl t-butyl ether to the solution, adding oxalic acid in ethyl acetate or methyl t-butyl ether to the dissolved naloxol-polyethylene conjugate over a period of at least 2 hours to produce a slurry and filtering the slurry to yield the oxalate salt in solid form.