& Key message In Mexican oaks, germination increases with acorn fresh weight across oak species. Within species, these relationships are stronger in red oaks than in white oaks. In both oak groups, fresh weight of acorns increases with their dry biomass. & Context Mexican oaks are phylogenetically grouped in red and white oaks. White oaks produce heavier acorns than red ones, but no studies have assessed whether this leads to different germination patterns. & Aims This study was aimed to determine the influence of the fresh weight of acorns on their germination. & Methods Acorns of red and white oaks were hydrated, weighed, and sowed under greenhouse conditions to assess whether their fresh weight was related with germination. We also assessed whether fresh weight of acorns was related with their dry biomass and/or water accumulation capability. & Results Hydrated acorns of white oaks were heavier and germinated faster than those of red oaks. Germination percentages increased with acorn fresh weight across oak species. Within species, germination probability increased with acorn fresh weight, but these relationships were more marked in red oaks. Germination speed decreased with acorn fresh weight in red oaks, but these relationships were not found in white oaks. Fresh weight was positively related with acorn dry biomass in all oak species, but it was not related with water content. & Conclusion Increasing acorn fresh weight enhances germination across oak species. Within species, however, this trait seems to have more influence in red than in white oaks.