This study investigated whether rainfall partitioning into throughfall, stemflow, and interception changes along a tree species diversity gradient. The 12 study plots in old-growth forest stands in the Hainich National Park, Germany, were composed of up to 11 tree species. Fagus sylvatica (beech) formed the monospecific plots. Mixed forest plots consisted of a variable admixture of other broad-leaved deciduous species such as Tilia spec., Fraxinus excelsior, Carpinus betulus, and Acer pseudoplatanus. Rainfall partitioning was influenced by several stand characteristics. Tree species diversity expressed as Shannon index was the variable that explained throughfall for different seasons most frequently. For example, in high-rainfall summer 2007, median throughfall per stand was between 66 and 77% of gross precipitation, whereas stemflow played a minor role (2–6% per stand). Throughfall correlated positively with Shannon index (rs = 0·74, p = 0·008), stemflow negatively (rs = − 0·87, p < 0·001), and interception showed no correlation along this beech to mixed forest gradient. These relationships were similar in summer 2005 and autumn 2006, but no or weak changes of throughfall with tree diversity were observed during other study periods. Multiple linear regressions supported the assumption that combinations of several characteristics are important. Shannon index in combination with mean dbh explained much of the variability observed in throughfall among stands in two seasons (up to R2adj = 0·63, p < 0·01). Influential stand characteristics varied between seasons and years due to different rainfall conditions. Spatial variability of throughfall within a stand did not change consistently with any stand characteristic. Copyright © 2009 John Wiley & Sons, Ltd.