Glycogen branching enzymes (GBEs; 1,4-α-glucan branching enzyme; E.C. 188.8.131.52) have so far been described to be capable of both α-1,6-transglycosylation (branching) and α-1,4-hydrolytic activity. The aim of the present study was to elucidate the mode of action of three distantly related GBEs from the glycoside hydrolase family 13 by in depth analysis of the activity on a well-defined substrate. For this purpose, the GBEs from R. marinus (RmGBE), P. mobilis (PmGBE1), and B. fibrisolvens (BfGBE) were incubated with a highly pure fraction of a linear substrate of 18 anhydroglucose units. A well-known and characterized branching enzyme from E. coli (EcGBE) was also taken along. Analysis of the chain length distribution over time revealed that, next to hydrolytic and branching activity, all three GBEs were capable of generating chains longer than the substrate, clearly showing α-1,4-transglycosylation activity. Furthermore, the GBEs used those elongated chains for further branching. The sequential activity of elongation and branching enabled the GBEs to modify the substrate to a far larger extent than would have been possible with branching activity alone. Overall, the three GBEs acted ambiguous on the defined substrate. RmGBE appeared to have a strong preference towards transferring chains of nine anhydroglucose units, even during elongation, with a comparably low activity. BfGBE generated an array of elongated chains before using the chains for introducing branches while PmGBE1 exhibited a behaviour intermediate of the other two enzymes. On the basis of the mode of action revealed in this research, an updated model of the mechanism of GBEs was proposed now including the α-1,4-transglycosylation activity.