Biochar produced by pyrolysis of organic residues is increasingly used for soil amendment and many other applications. However, analytical methods for its physical and chemical characterization are yet far from being specifically adapted, optimized, and standardized. Therefore, the COST Action TD1107 conducted an inter-laboratory comparison, in which 22 laboratories from twelve countries analyzed three different types of biochar for 38 physical-chemical parameters (macro- and micro-elements, heavy metals, polycyclic aromatic hydrocarbons, pH, electrical conductivity, and specific surface area) with their preferential methods. The data was evaluated in detail using professional inter-laboratory testing software. Whereas intra-laboratory repeatability was generally good or at least acceptable, inter-laboratory reproducibility was mostly not (20% < mean reproducibility standard deviation < 460%). This paper contributes to better comparability of biochar data published already, and provides recommendations to improve and harmonize specific methods for biochar analysis in the future.