A newly designed high-pressure combustion facility was used to study the structures and extinction conditions of counterflow diffusion flames in air for nitrogen-diluted methane, ethane, and ethylene, from 0.1 MPa to 2.0 MPa. Besides employing thermocouples to measure temperature profiles, strain rates at extinction were measured and compared with predictions of two different chemical-kinetic mechanisms (San Diego and USC). In addition, the nitrogen in the fuel and oxidizer streams was replaced by helium for one of the methane tests of extinction strain rate as a function of pressure. In all cases, the strain rate at extinction was found to increase with pressure up to about 0.3-0.5 MPa and to decrease with pressure thereafter, on up to 2.0 MPa, although with helium there was a clear leveling tendency beyond 1.0 MPa. While these behaviors were in qualitative agreement with most predictions of the chemical-kinetic mechanisms, in a number of cases the quantitative discrepancies were well beyond the experimental uncertainty. This underscores the desirability of improving chemical-kinetic descriptions for applications at elevated pressures. Such improvements for the San Diego mechanism are introduced here for two of the steps involving hydroperoxyl that become increasingly important with increasing pressure. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.