Ash deposition causes serious problems for the operation of solid fuel-fired boilers. In this paper, we measured the sticking probability of ash particles at elevated temperatures based on a diffusive flat-flame burner. The compositions, weight losses, melting behaviors and the sticking probabilities as functions of the temperature were investigated for ash particles from the combustion of various solid fuels (low-rank coal + kaoline, biomass, co-firing coal with sludge). It was shown that the slags produced by Zhundong coal with kaoline additives and biomass are rich in silicon and aluminum, and the sticking probabilities gradually increase with the temperature; By contrast, the slag produced by cofiring coal with sludge is rich in calcium, and a sharp increase of the sticking probability is observed when the temperature rises above 1150◦C. In addition, existing model predictions of sticking probability, based on either melt fraction or viscosity, deviate greatly from the experimental results, and thus cannot be used for practical purposes. The experimental methods and measurement results in this study are beneficial for future work on ash sticking probability models at elevated temperatures.