Preparation of aldehydes and/or alcohols having from 6 to 30 carbon atoms

A process for preparing aldehydes and/or alcohols having from 6 to 30 carbon atoms by hydroformylation of olefins by means of synthesis gas in the presence of a catalyst at from 120° C. to 210° C. and pressures of from 100 to 400 bar, comprising introducing a reaction mixture comprising olefins, synthesis gas and catalyst or catalyst precursor at high velocity, into a high-pressure reactor via an inlet nozzle having an adjustable flow cross section, monitoring the throughput of the reaction mixture thought the inlet nozzle, maintaining a constant exit velocity of the reaction mixture from the inlet nozzle into the high-pressure reactor by adjusting the flow cross section of the nozzle.