The present invention provides PEGylated hemoglobins and PEGylated albumins comprising polyethylene glycol (PEG) conjugated to hemoglobin or to albumin, wherein the PEG is a maleimide PEG, an alkylamide PEG, an iodoacetamide PEG, a p-nitro thio-phenyl PEG, a vinyl sulfone PEG, or a mixed disulfide PEG and PEGylated albumins and PEGylated hemoglobins comprising polyethylene glycol (PEG) attached to a thiolated amino group of albumin or hemoglobin, wherein the amino group is thiolated using dithio sulfo succinimidyl propionate (DTSSP) or dithiosuccinimidyl propionate (DTSP) or dithiobispropionimidate. The invention also provides methods of preparing PEGylated hemoglobins and PEGylated albumins comprising a) reacting hemoglobin or albumin with a thiolating agent and with a PEGylating agent, and b) capping unPEGylated reactive thiols of hemoglobin or albumin with N-ethyl maleimide.
