UH 25 is a fuel mixture for rockets. It was developed for the European Ariane

Ariane (rocket)

Ariane is a series of a European civilian expendable launch vehicles for space launch use. The name comes from the French spelling of the mythological character Ariadne....

 2-4 launch vehicles.

UH 25 was developed after a disaster during flight 2 of the Ariane 1 rocket. During launch, one of the four Viking

Viking (rocket engine)

The Viking rocket engines were members of a series of bipropellant engines for the first and second stages of the Ariane 1 through Ariane 4 commercial launch vehicles, using storable, hypergolic propellants, N2O4/UH 25...

 engines on the first stage developed a combustion instability which lead to an engine fire, subsequent explosion and destruction of the vehicle. Following this event and starting with Ariane 2, the fuel was changed from pure UDMH to the mixture UH 25.

UH 25 was used in Ariane rocket versions 2 through 4.

UH 25 is a mixture of 75% UDMH

UDMH

Unsymmetrical dimethylhydrazine is a toxic volatile hygroscopic clear liquid, with a sharp, fishy, ammoniacal smell typical for organic amines. It turns yellowish on exposure to air and absorbs oxygen and carbon dioxide. It mixes completely with water, ethanol, and kerosene. In concentration...

and 25% hydrazine hydrate. It is hypergolic with dinitrogen tetroxide

Dinitrogen tetroxide

Dinitrogen tetroxide is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide; some call this mixture dinitrogen tetroxide, while some call it nitrogen dioxide.Dinitrogen tetroxide is a powerful oxidizer, making it highly...

as oxidizer, and both can be stored as liquids at room temperature.

Safety labels

T toxic	F easily flammable