Icelandic downy-birch (Betula pubescens) is often shrub-like and polycormic compared to the same species in Europe. This type of birch is relatively common in the subarctic and mountainous regions of Europe and hence has been called mountain-birch (B. pubescens ssp. tortuosa). The morphological characteristics of mountain-birch are thought to reflect past hybridization between downy-birch and dwarf-birch (B. nana). These species have different chromosome numbers: downy-birch is a tetraploid species with 56 chromosomes, whereas dwarf-birch is diploid with 28 chromosomes. Triploid hybrids of the two species, bearing 42 chromosomes, have been found in a relatively high frequency in natural woodlands in Iceland. Birch is wind-pollinated and produces large quantities of pollen. Pollen grains are well preserved in wet and damp places and therefore pollen in lake sediments and peat is widely used to reveal history of vegetation and climate. Differentiation of downy-birch and dwarf-birch pollen is difficult but important in the interpretation of past ecology. We measured pollen from 39 downy-birch, 31 dwarf-birch and 22 triploid plants in order to determine its relative size and shape. The difference in pollen size between the two birch species has turned out to be less than those published elsewhere, probably because the effects of introgressive hybridization between the two species become clearer when species identification is based on cytotaxonomy rather than morphology. We also found distinctive characteristics linked to hybrid pollen, which was often damaged or malformed. The results can be used to track the history and origin of birch in Iceland.