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Bioenergy production from grassland biomass is an alternative use for semi-natural grasslands, but chemical composition of grassland herbage can limit its suitability for combustion. Leaching by precipitation during the field period is currently used to remove unwanted elements from other herbaceous biofuels. The aims of this study were to assess the influence of leaching on chemical composition of grassland biomass, to establish the influence of leaching intensity and to evaluate the feasibility of a simplified laboratory method for leaching studies. Freshly cut and dried herbage from two contrasting grasslands were leached using 12 and 36 mm of simulated rain. A second experiment compared three methods of different leaching intensity. Leaching resulted in strong reduction in Cl and K concentrations; ash was moderately reduced and Mg slightly reduced. Concentrations of N, S and Ca were not affected. Leaching occurred in dried, but not in fresh herbage, and Cl leaching was affected by herbage type. Cl, K and ash responded differently to increased leaching intensity. Slagging, fouling and corrosion problems during combustion, which are closely linked to Cl and K concentrations, could be reduced by natural leaching under suitable weather conditions. A standardized laboratory leaching method is suggested for further investigating the influence of herbage characteristics on the leachability of K and Cl.