Abstract Past evidence shows that the secondary structure of photosynthetic Reaction Centres is preserved in Langmuir-Blodgett films at temperatures of up to 473 K, and that the corresponding structure in solution is lost at 328 K. This evidence is correlated with findings in this study that the functional activity of Reaction Centres is preserved up to 368 K in Langmuir-Blodgett films but only up to 308 K in dried random smears (non-oriented films). Interestingly, a similar behaviour is displayed by anti-insulin antibodies in Langmuir-Blodgett films. The role of dehydration in the preservation of secondary structure is investigated using the latter protein, and, upon comparison with the role of close packing is found to be a secondary effect. The preservation of antibody functional properties is monitored up to 423 K and unique catalytic properties appear after thermal treatment. This phenomenon is studied by making surface potential measurements and is found to be caused by the increased anisotropy of the antibody layer after thermal treatment. This increased anisotropy, related to the enhanced order of the film due to thermal treatment followed by slow cooling, is confirmed by X-ray analysis. The above evidence is collected in a unitary picture that attributes the cause of protein thermal stabilization to the close packing induced by Langmuir-Blodgett film formation; the decreased conformational entropy is implicated as the initial condition that allows protein recrystallization after thermal treatment.