It is shown that successive ionic layer deposition (SILD) method can be used for a surface functionalizing of the SnO₂ films and improving their gas sensing properties. It was found that gold clusters deposited on the SnO₂ surface were active to both reducing and oxidizing gases. However, the most significant increase in the conductivity response was observed during detection of ozone. It was established that the effect of the SnO₂ surface decoration by AuNPs on gas sensing characteristics depends on the number of deposition cycles, i.e. the size of gold clusters. Moreover, there is an optimal level of surface doping, and it corresponds to a very small degree of the surface coverage by gold. Models showing the promotional role of Au additives were then discussed, and the mechanism of sensitization in the SnO₂:Au-based gas sensor are proposed.