The impact of geology, climate, soil type and vegetation on forest humus forms was studied in the Veneto Region (northern Italy). A total of 352 study sites were compared by Redundancy Analysis (RDA). Humus forms were described by the structure (micro-, meso-, or macro-aggregated) of the organo-mineral A horizon, by the thick- ness of litter horizons and by their nomenclature, which followed the morpho-functional classification recently proposed for inclusion in the WRB-FAO. The size of aggregates within the A horizon was distributed along a com- mon gradient embracing geology, climate, soils and vegetation. Macro-aggregation (as opposed to micro- aggregation, meso-aggregation being intermediate) was favored by carbonated (as opposed to silicated) parent rocks, warmer climate associated to lower elevation, lower soil acidity, deciduous (as opposed to coniferous) for- est vegetation and relatively high clay content. The amphi group of humus forms, associated with carbonated substrates in Esalpic and Mesalpic climate districts, was distributed according to thickness of litter horizons along a gradient of soil stoniness. Biological reasons for the observed environmental influences on the size of soil aggregates, a criterion of humus form classification, were discussed to the light of knowledge on annelid (earthworm and enchytraeid) ecology. Humus forms can be easily identified and classified on the field, using a table included in the article. Our results can be used for mapping the distribution of forest humus forms in the Veneto Region, implying a better understanding of carbon cycling processes in the frame of present-day global warming.