This work presents the first non-intrusive quantitative measurements of [OH] and temperature in H-2/CO syngas/air counterflow diffusion flames as a function of H-2:CO ratio and local strain rates. The H-2:CO ratio was varied as 8:32, 20:20 and 32:8 volumetrically maintaining a constant dilution of 58% N-2 and 2% CH4. The local strain rates ranged from 35 s(-1) to 1180 s(-1). Peak temperature and [OH] were observed to increase with increasing H-2:CO ratio in the syngas fuel. Peak [OH] displayed a non-monotonous function of strain rate for all compositions of syngas. The measurements were used for rigorous assessment of five H-2/CO chemical kinetic mechanisms from the literature designated as M1 to M5. The M1 mechanism was found to provide the best overall match with experiments, however, significant discrepancies were observed in comparison between measurements and [OH] predictions for low H-2:CO ratio at low strain rates and for higher H-2:CO ratio at high strain rates. While sensitivity analyses and reaction rate analyses identified H + O-2 + M double left right arrow HO2 + M as one of the key reactions affecting the [OH] at low strain rates. However, at higher strain rates, chain-branching reactions such as H + O-2 double left right arrow OH + O and O + H-2 double left right arrow OH + H were found to be important at higher strain rates. The reaction sensitivity towards [OH] was found to change from negative to positive as the strain was increased for medium and high H-2:CO ratios. A reduction in the Arrhenius co-efficient for the third body reactions H + 02 + M + HO2 + M and H + OH + M double left right arrow H2O + M showed significant improvements in the predictions of [OH] and extinction limits for non-preniixed flames while showing good agreement with the laminar flame speed data available in the literature. (C) 2016 Elsevier Ltd. All rights reserved.