When terrestrial mosses are used to study the risks associated with the accumulation of airborne pollutants, samples must be washed to remove deposits adhering to the surface. Here, we studied the effects of washing the samples as well as of freezing, drying, and acclimatization treatments. The differences between the concentrations of Al, As, Cd, Cr, Cu, Fe, Hg, K, Ni, Pb, Se, V, and Zn in washed and unwashed samples were significant, and therefore, it would not be valid to combine or compare the data corresponding to samples treated thus. We also investigated whether there were significant differences in the concentrations of various metals and metalloids yielded from samples subjected to different treatments prior to washing (freezing, drying, acclimatization after drying, and acclimatization only), while also considering possible effects of the level and type of contamination and the weather conditions. For many of the elements, we found significant differences that depended on both the levels of contamination at each sampling site and the weather. As these differences may be due to changes in the contents of the different cellular fractions, the sequential elution technique was applied to determine the locations of Cu, K, and Zn. The freezing and drying procedures were rejected because they alter the permeability of the membranes and lead to underestimation of the concentrations of elements. With the acclimatization procedure, particles of some elements adhere to the surface, making this option unsuitable when a description of bioconcentration is required. Taking into account these limitations and the lack of an optimal procedure, acclimatization after drying is proposed as the best option for harmonizing the method.