Massbalancesof ash and potassium forafluidized bed combustor were performed incorporating measurement uncertainties. The total outputmassof ash orachemical element should be equaltothemassin the input fuel; however, this is not often achieved.Arealisticestimationof recoveryuncertaintycan support the reliability ofamassbalance.Estimationofuncertaintyhelpstoestablishareliable evaluation of the recovery ratio of ashmassand elementalmass. This may clarify whether any apparent lack in closing themassbalance can be attributedtouncertainties. The evaluation of measurementuncertaintyfor different matrices, namely coal, biomass, sand and ashes from different streams was basedoninternal quality control data and external quality control data, namely analysis of samples from proficiency tests or use ofacertified reference material. The evaluation of intermediate precision and trueness allowed theestimationof measurementuncertainty. Duetothe different physic and chemical characteristics of the studied matrices, theuncertaintyof precision was evaluated using R-charts of data obtained from the analysis of duplicates for the majority of samples. This allowed evaluating sample heterogeneity effects. The instrumental acceptance criterion was also considered and included in the combineduncertainty. The trueness was evaluated using data from several proficiency tests and from analysis ofacertified reference material or sample spiking. Statistically significant bias was included.