Oak wood leachate obtained from two storage facilities (storage pound and ditch) in a wood-based industry, and leachate generated by a laboratory leaching test, were characterized in seven categories regarding particle size distribution (PSD) (raw leachate, ≤ 20 μm, ≤ 10 μm, ≤ 1.2 μm, ≤ 13 nm, ≤ 5 nm and ≤ 2 nm). The PSD followed a normal distribution model with a correlation coefficient (r) varying from 82 to 88. Each fraction was analysed regarding chemical oxygen demand, polyphenols and acute toxicity in toxicity assays with Artemia salina, Vibrio fischeri and Lactuca sativa. Fractions with particles >1.2 μm were more toxic to A. salina and V. fisheri than fractions with particles ≤ 1.2 μm. No toxic effect was observed for L. sativa. The results suggest that polyphenols are the main toxic compounds in oak wood leachate. A conspicuous difference was found between field and laboratory samples.