Prerequisite for the use of Citybike Wien is a one-time registration. You can do it at a station or right here online.
As soon as registered you can rent a bike at any Citybike station in Vienna. Around the clock, the whole year.
Returning a bike is easy: just insert your bike into an empty bikebox at any Citybike Wien station. The green light on the bikebox confirms the successful return of the bike.