This work presents the first non-intrusive quantitative measurements of [OH] and temperature in H2/CO syngas/air counterflow diffusion flames as a function of H2:CO ratio and local strain rates. The H2:CO ratio was varied as 8:32, 20:20 and 32:8 volumetrically maintaining a constant dilution of 58% N2and 2% CH4. The local strain rates ranged from 35 s−1to 1180 s−1. Peak temperature and [OH] were observed to increase with increasing H2: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 H2/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 H2:CO ratio at low strain rates and for higher H2:CO ratio at high strain rates. While sensitivity analyses and reaction rate analyses identified H + O2+ M ⇔ 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 + O2⇔ OH + O and O + H2⇔ 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 H2:CO ratios. A reduction in the Arrhenius co-efficient for the third body reactions H + O2+ M ⇔ HO2+ M and H + OH + M ⇔ H2O + M showed significant improvements in the predictions of [OH] and extinction limits for non-premixed flames while showing good agreement with the laminar flame speed data available in the literature.