Abstract <p>Fire-derived charcoal is a common product in postfire forest ecosystems. Its role in litter decomposition is still poorly understood, and even less is known about the relationship between charcoal and soil fauna. In a field “litterbag” experiment using the physical exclusion of fauna by means of mesh of different sizes, the influence of charcoal on microbial activity indices at the early stage of oak leaf decomposition under conditions of faunal community complexity has been studied. The response of microorganisms to the addition of charcoal and faunal community complexity was evaluated using the substrate-induced respiration method in terms of the microbial respiration coefficient (BR/SIR) (basal respiration/substrate-induced respiration), the microbial metabolic coefficient (<i>q</i>CO₂), and microbial biomass carbon (С_MB). When the faunal community complexity increased from microfauna to mesofauna, regardless of the presence of charcoal, microbial biomass increased by 40%. In the control treatment, a 21% decrease in <i>q</i>CO₂ and BR/SIR was observed with the access of mesofauna compared to access of microfauna alone. In the charcoal treatment, the decrease in <i>q</i>CO₂ and BR/SIR in the presence of mesofauna was twice as pronounced and amounted to 44% (<i>p</i> &lt; 0.05), and an acceleration of mass loss by 17% was also observed. In our study, the decrease in <i>q</i>CO₂ in the presence of mesofauna and its even greater decrease in the charcoal–mesofauna treatment were due to an increase in microbial biomass, but not to an increase in BR. The further complication of the faunal community (access of macrofauna) did not lead to significant changes in microbial activity indices, irrespective of the addition of charcoal.</p>