The formation of poly(ethylene glycol) (PEG) hydrate during freeze-thawing of dilute lactate dehydrogenase solutions with the addition of 0.05-160 mg/mL PEG 6000 is investigated by differential scanning calorimetry and modulated temperature differential scanning calorimetry. The freeze-thawing process is performed with a controlled temperature history. A moderate cooling rate to a low freezing temperature in combination with a low heating rate seems to create the most stable PEG hydrate. The maximum amount and the most stable hydrate phase are obtained when the freezing temperature is at or below -60degreesC. The enthalpy of melting for the hydrate at -15degreesC is dependent on the heating rate but not on the cooling rate if the freezing temperature is -60degreesC. The effect of the addition of reduced form nicotinamide adenine dinucleotide to the PEG and protein solution indicates that competing interactions with the protein can increase the stability of the PEG hydrate. The amount of bound water in the PEG hydrate can be calculated directly from the melting enthalpy of the hydrate if an adequate temperature history is used. For solutions with >10 mg/mL PEG there are 1.7-2.7 water molecules bound per PEG unit. The PEG protection of the protein at freeze-thawing can be an effect of the amount of available PEG hydrate in relation to the amount of ice surface. (C) 2003 Wiley Periodicals, Inc.