The modulatory effect of oligoguluronate (GB) on the gelation of macromolecular alginate (ALG) was investigated by means of dynamic oscillatory rheology, fluorescence and AFM measurements. An inhibitory effect in the range of 0.25 < R (Ca/G) < 0.60 and a promotive effect in the range of R > 0.60, were quantified. The two effects are thought to be associated with the different molecular events that dominate the gelation of alginate, namely, egg-box dimerization and lateral aggregation. Quantitative analysis indicates a competitive binding rather than a screening binding during egg-box dimerization, which is responsible for the inhibitory effect in the lower Ca concentration regime. On the other hand, in the higher Ca concentration regime where alginate gelation is predominated by chain lateral aggregation, the dimers formed by GB could act as a binder to enhance the aggregation of alginate dimers, resulting in a promotive effect on alginate gelation. The results are consistent with the microstructures observed by AFM. (C) 2015 Elsevier Ltd. All rights reserved.