Simultaneous assessment of epiphytic lichen diversity by teams from six European countries, following the procedures defined in the recently adopted European standard, revealed several sources of error (e.g. location of plot, selection of trees, identification of taxa). Routine training and further improvement of Standard Operating Procedures (SOPs) are suggested to boost the harmonization process of the European guidelines, which promises to be an effective tool for standardizing the assessment of lichen diversity. © 2014 Elsevier Ltd.