The influence of the naturally occurring osmolytes xylitol, glycine and betaine on the thermal stability of human haemoglobin was investigated. Experiments were made in the temperature range of 55-70°C, adding up to 30% w/w of osmolytes to the protein solution. All the additives stabilized haemoglobin, with xylitol and glycine appearing more effective. A kinetic analysis based on the Lumry-Eyring inactivation scheme showed that the denaturation process can be described by a second-order rate expression, with an apparent activation energy ranging from 45 to 82 kcal/mol. © 2002 Elsevier Science B.V. All rights reserved.