Ash deposition, fouling, and slagging are commonly undesirable occurrences in coal utility boilers and gasifiers. Ash deposit formation is usually initiated by sintering of the particles. The sintering characteristics of the ash have to be evaluated to understand the process of deposit formation. Cold compression strength (CCS) tests on heat treated ash pellets produced under defined conditions can be applied for the determination of the initial sintering temperature (IST). Scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDX), high temperature X-ray diffraction analysis (HT-XRD), and thermogravimetric differential scanning calorimetry (TG-DSC) are analytical methods to characterize the mineral transitions responsible for sintering. CCS tests were done on ashes of three different coals to determine their IST. For this purpose ash pellets were pretreated at different temperatures. The influence of oxidizing, inert, and reducing conditions at ambient pressure was investigated. The IST were compared with the characteristic temperatures from ash fusibility tests (AFT). HT-XRD, TG-DSC, and SEM/EDX were applied for a more detailed characterization of sintering. The IST determined by CCS tests are considerably lower than the respective initial deformation temperature (IDT) obtained by AFT. Thus, the AFT is not suitable to detect IST. Major mineral transitions were found which are responsible for initial sintering in the case of the investigated ashes. The formation of silicates or the crystallization of iron-aluminum-oxides were found to induce sintering at oxidizing and inert conditions. Sintering at reducing conditions was initiated by carbonation of calcium compounds, silicate formation, or melting of iron sulfide. These phases can be understood as the relevant phases for initial strengthening of fouling deposits in coal utility boilers and gasifiers in the case of the investigated ashes. (C) 2017 Elsevier Ltd. All rights reserved.