In this article, we describe a new method to obtain D₂O/H₂O exchange in photosynthetic reaction centres from Rhodobacter sphaeroides. The method is characterized by: (i) a very high efficiency of the isotopic replacement; (ii) an extremely low amount of D₂O needed; (iii) the short time required for dehydration and D₂O rehydration; (iv) the possibility of controlling concomitantly the hydration state of the sample. The proposed method can be applied to other proteins.