Use of reduced chemical mechanisms in combustion applications is becoming standard practice due to the reduced computational effort involved in simulating finite rate chemistry. Reduced mechanisms are developed with simplifying assumptions which limit the phenomena that can be studied. In this study, a comparison between a full, finite rate, chemical mechanism and a standard reduced mechanism for H-2-air combustion and NO formation is made for nonpremixed, turbulent jet flames. The numerical simulations incorporate an innovative mixing model so that turbulent-chemistry interactions can accurately be captured enabling a critical evaluation of the assumptions made in the reduced mechanism. Results indicate that the reduced mechanism and the full mechanism with differential diffusion and variable Lewis number compare relatively well except for the radical species. NO emission index results also compare favorably even though substantial variations in Lewis number and differential diffusion effects are found to exist in the full mechanism results.