Method of grouping sheet units, which belong together in sheet groups, of a sheet stream comprising two superposed sheet units each, comprising the following steps: a) transferring the superposed sheet units into a grouping unit ; b) transferring the superposed sheet units into a collecting station disposed downstream of the grouping unit if the sheet units transferred into the grouping unit belong to the same sheet group; c) otherwise: transferring a sheet unit into the collecting station and holding the other sheet unit back in the grouping unit , with the following provisos: i) transferring the held-back sheet unit into the collecting station if this sheet unit is not part of a sheet group of one of the sheet units to be transferred next into the grouping unit ; ii) transferring the held-back sheet unit into the collecting station together with both sheet units to be transferred next into the grouping unit if both sheet units to be transferred next into the grouping unit belong to the sheet group of the held-back sheet unit; iii) otherwise: transferring the held-back sheet unit together with the sheet unit belonging to the same sheet group of the sheet units to be transferred next into the grouping unit and holding the other sheet unit back in the grouping unit and continuing the method with one of the method steps i) to iii). a) transferring the superposed sheet units into a grouping unit ; b) transferring the superposed sheet units into a collecting station disposed downstream of the grouping unit if the sheet units transferred into the grouping unit belong to the same sheet group; c) otherwise: transferring a sheet unit into the collecting station and holding the other sheet unit back in the grouping unit , with the following provisos: i) transferring the held-back sheet unit into the collecting station if this sheet unit is not part of a sheet group of one of the sheet units to be transferred next into the grouping unit ; ii) transferring the held-back sheet unit into the collecting station together with both sheet units to be transferred next into the grouping unit if both sheet units to be transferred next into the grouping unit belong to the sheet group of the held-back sheet unit; iii) otherwise: transferring the held-back sheet unit together with the sheet unit belonging to the same sheet group of the sheet units to be transferred next into the grouping unit and holding the other sheet unit back in the grouping unit and continuing the method with one of the method steps i) to iii). i) transferring the held-back sheet unit into the collecting station if this sheet unit is not part of a sheet group of one of the sheet units to be transferred next into the grouping unit ; ii) transferring the held-back sheet unit into the collecting station together with both sheet units to be transferred next into the grouping unit if both sheet units to be transferred next into the grouping unit belong to the sheet group of the held-back sheet unit; iii) otherwise: transferring the held-back sheet unit together with the sheet unit belonging to the same sheet group of the sheet units to be transferred next into the grouping unit and holding the other sheet unit back in the grouping unit and continuing the method with one of the method steps i) to iii). <EOS>