The structure and protonation of the complexes trans-[(ReO2L2)-O-V](+) (L = tmen (N,N,N;,N;-tetramethylethylenediamine), deten (N,N-diethylethylenediamine) and dieten (N,N'-diethylethylenediamine)) were studied. The structure of [ReO2(dieten)(2)]I center dot 2H(2)O shows a trans- dioxo geometry (Re-O average distance 1.766(5) angstrom) and an equatorial plane defined by four N atoms belonging to two bidentate amines (Re-N average distance 2.190(7) angstrom). The Re atom lies on an inversion centre. These complexes can be protonated in aqueous solution and the protonation constants have been determined at 25 degrees C and I = 0.5 M KCl. The protonated species [ReO(OH)(deten)(2)](ClO4)(2) was isolated and characterized by X-ray diffraction. The squashed octahedral geometry is maintained around the Re( V) atom ( which also lies on an inversion centre) with larger Re-O distances (average 1.780(6) angstrom). By the use of N-alkylated diamines as equatorial ligands, complex protonation is confined to a low pH range (pH < 3) and one oxo group is preserved even in very acidic media.