The measured volatile suspended solids (VSS) fraction of activated sludge is assumed to represent the organic fraction of suspended solids. However, mineral suspended solids can also lose some mass at 550C. Experiments were performed on iron phosphate and hydroxide precipitates, which form upon ferric addition into the secondary stage for phosphorus removal. They showed that the precipitates lose 6 to 18% of their mass during VSS measurement. Suggested explanations are loss of bound water and partial chemical decomposition involving gas formation. Experiments showed that the extent of mass loss will depend on actual temperature and duration of heat exposure during the VSS measurement, as well as implementation of a washing step to remove dissolved solids from the mass retained on the filter. VSS thus overestimate the truly organic fraction of sludge. The error will increase with the mineral content of the sludge, which depends on influent characteristics, chemical doses and sludge retention time (SRT). Two case studies show that if several of these factors occur in combination on the plant, the error will be significant compared to the VSS measurement error. If the measured sludge VSS fraction is used in calculations, the actual F/M ratio and SRT will be underestimated and the actual sludge wastage rate will be overestimated.