The invention relates to a method and device (100 or 100') for providing at least one fuel, in particular, for motor vehicle engines. The invention enables fuel to be produced, in particular, in the form of hydrogen, according to needs, and in a simple, economical, clean and safe manner and, furthermore, preferably on-board a means of transportation. According to the invention, at least one mixture consisting of water (H.sub.2) and ammonium nitrite (NH.sub.4NO.sub.2) and/or ammonium nitrate (NH.sub.4NO.sub.3), which serve(s) as a reaction constituent, is introduced as a fuel or fuel mixture into at least one, in particular, second reaction space (40'). In addition, en air (Lu), oxygen (O.sub.2) and/or ozone (O.sub.3) are supplied to this mixture, which is located in the, in particular, second reaction space (40') and rendered unstable due to charge transfer.