Bio-monitoring with mosses is a convenient tool to establish specific or large-scale exposure to 21 atmospheric trace metals and nitrogen pollution. However, the uncertainty associated with the 22 sampling, sample preparation and chemical analysis of bio-monitors is poorly documented, except one 23 study dealing with lichens, so that uncertainty associated with moss bio-monitoring has never been 24 assessed. Here, we propose to follow the Eurachem guidelines to determine the uncertainty associated 25 with the concentrations of elements measured in mosses in one sampling survey over France in 2011, 26 plus a focus on the analytical method for four surveys from 1996 to 2011. Uncertainties were expressed as linear functions of the concentrations and their slope ranged from 14 to 61% for nitrogen and chromium, with a median of 49%. Although data reveal some points of the protocol to carefully perform, they indicate its proper execution and reproducibility. The chemical analyses contribute for a small part to the uncertainty associated with the protocol, excepted chromium. The sampling period as well as the intra-site variability are largely involved in this uncertainty. The moss species induce a supplementary uncertainty only for calcium, cadmium, copper, and iron. This integrative assessment of the measurement uncertainty will help improve the protocol for future surveys. Such studies should help to propose a standard operating procedure, and ease and improve comparisons between countries and the study of temporal trends over Europe.