The acid-base characteristics of troilite mineral are investigated at 25 degrees C and ionic strength between 0 and 0.1 M NaCl by potentiometric titration, solubility measurements and quantum chemical calculations. Constant capacitance model with one amphoteric surface site is used to interpret the potentiometric titration data. It is found that the point of zero charge (pH(PZC)) decreases with increasing the ionic strength. The model proposed for the description of this behavior indicates that pH(PZC) may vary with the ionic strength even if the structural charges equal zero. The quantum chemical calculations show that the stability of the studied surface species increases in the order of FeS < FeSH < FeSH2. It has also been found that the positive charge of surface-bound hydrogen atoms is very low (< +0.1). This small positive charge of hydrogen at troilite surface could explain the adsorption of cations at the mineral surface, even at low pH values.