In this study, organic matter mineralization was studied in the Quercus coccifera (Kermes oak, Fagaceae) soils mixed with Quercus leaves having high tannin content, Melia azedarach (Neem tree, Meliaceae) leaves and fruits having high azadirachtin content, pure tannin and pure azadirachtin. Carbon mineralization of all samples was determined by the CO2, respiration method over 30 days (28 degrees C and 80% of field capacity). Addition of equal (1x) and half (1/2x) amounts of soil carbon containing leaves of both plants to soils separately increased microbial activity The increase in microbial activity was higher in 1x leaves added soil compared with 1/2x leaves added soil and was only significant in lx Melia leaves added soil compared with control (P<0,001). There was no significant difference in carbon mineralization between the 1/2x and lx Quercus leaves added soils (P >= 0.05) whereas the difference between the Melia leaves added (1/2x; lx) soils was significant (P <0,001). It was determined that the carbon mineralization rates (%) of Quercus soil mixed with pure tannin and pure azadirachtin showed similar behaviours. When amounts of pure tannin and pure azadirachtin were increased, this rates reduced. Based on the results obtained from the findings of this study, it was determined that microorganisms used tannin and azadirachtin as carbon sources and preferred to use azadirachtin rather than tannin.