Abstract: The invention relates to a process and an apparatus for gasifying liquid and/or fine-grain solid gasification substances and/or for reforming a gas, using a gasification agent, in a reaction (1). In the process the process heat is supplied by heat carrier particles which are heated within a substantially closed circuit in a heater (5) by combustion gases which are produced in a combustion chamber (3), and passed through the reactor (1) in counter-flow relationship with the gasification substance or the gas to be reformed and the gasification agent, and then returned to the heater (5) for renewed heating. In accordance with the invention the particles and the combustion gas form a fluidized bed above at least one grid (34, 34a, 34b, 34c, 34d, 34e) arranged in the heater (5).