A method for growing a thin film on a substrate. A substrate is placed in a reaction space. The substrate is subjected to at least two vapor phase reactants in the form of vapor phase pulses, repeatedly and alternately. Gas within the reaction space is purged between two successive vapor phase pulses essentially entirely by use of a pump connected to the reaction space. The reaction space is purged between two successive vapor phase pulses such that less than 1% of the residual components from the first vapor phase pulse remains prior to the inflow of the second vapor phase pulse.