The present work focuses on the surface tension of nanofluids. An empirical model of the surface tension of nanofluids and a mechanism for the influence of nanoparticles on the value of this property are proposed. The paper explains the processes causing an increase as well as a decrease in the surface tension of nanofluids and why the value of the surface tension changes with increasing nanoparticle concentration only within a certain range. A mathematical function is presented to model this behaviour and predict both the maximum value of the surface tension and the nanoparticle concentration at which this value is reached. For the verification of the model, experimental studies of the surface tension of four types of ethylene glycol - based nanofluids indium oxide (In₂O₃), titanium oxide (TiO₂), zinc oxide (ZnO) and multiwalled carbon nanotubes (MWCNT) at a temperature of 298.15K were carried out. The proposed model agrees well with the obtained surface tension values of the investigated nanofluids.