Laminar diluted hydrogen flames have been studied in a counterflow burner. Measurements were made of profiles of temperature and of [NO], which are compared with predictions from numerical computations. A constant strain rate of 100 s(-1) was used in every experiment. The hydrogen stream was diluted with the inerts N-2, CO2, and He. Besides diffusion flames, the fuel streams were partially premixed to equivalence ratios from 2.7 to 3.9. There is evidence that, for these degrees of premixing, attached to the exit of the fuel duct there is a rich premixed flame, whose hot products burn in a diffusion flame close to the stagnation plane. Differences between the influences of the various diluents are discussed in terms of differences in heat capacities, thermal-radiative properties, and effects on chemical reactions. Maximum measured flame temperatures varied from 1830 to 2170 K, and measured [NO] ranged from 3.6 to 21.5 ppm. Numerical predictions show good agreement with experiments for both temperatures and [NO] at the lower temperatures. At higher temperatures, the predicted [NO] exceeds its measured value, the difference being attributed to probe effects.