The effects of tree thinning on soil respiration and microbial respiration in a Hungarian oak (Quercus frainetto Ten.) forest were examined over a 2-year period (2010-12). Tree density was reduced to 50% of the basal area. The research focus was on the main factors influencing the soil respiration (R-S) and microbial respiration in the forest floor (R-FFM) and in the soil (R-SM): soil temperature, moisture, carbon (C), nitrogen (N), and pH; groundcover biomass (GC); forest floor mass, carbon and nitrogen; and fine root biomass. R-S was measured twice monthly with the soda-lime method, and the incubation method was used to measure R-SM and R-FFM separately. The results were evaluated annually and over the 2-year research period. Correlation and stepwise regression analyses were used for statistical evaluation. Annual mean R-S was significantly higher in thinned plots (1.92gCm(-2)day(-1)) than in the control plots (1.79gCm(-2)day(-1)). Over the 2-year research period, R-S was higher in the thinned plots, and had linear correlations with GC, soil temperature and fine root biomass. GC was found to be the main factor that determined R-S. The control plots had significantly higher R-SM in first year, whereas the thinned plots had significantly higher R-SM in second year; no significant difference was found over the 2-year research period. R-FFM was significantly higher in the control plots than in the thinned plots, by 84% in the second year and by 34% over the 2-year study period. R-SM had a linear correlation with soil N content and soil pH, whereas R-FFM had linear correlations with C concentration and the C:N ratio of the forest floor in the thinned plots.