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

UH 25

UH 25 is a fuel mixture for rockets. It was developed for the European Ariane 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 engines on the first stage developed a combustion instability which lead to an engine...

 (mixture 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

Hydrazine

Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

.)

The earliest versions, developed in 1973, had a thrust of about 390 kN.
The version used on the Ariane 4

Ariane 4

Ariane 4 was an expendable launch system, designed by the Centre National d'Etudes Spatiales and manufactured and marketed by its subsidiary Arianespace. Ariane 4 was justly known as the ‘workhorse’ of the Ariane family. Since its first flight on 15 June 1988 until the last, on 15 February 2003, it...

 first stage, which clustered four together, had 667 kN thrust each.