Abstract Counterflow configurations are useful for investigating the structures of premixed, non-premixed, and partially premixed flames. Ignition and extinction conditions also are readily measured in this configuration. There is a wide range of different possible designs of apparatus to be used in such measurements. The choices vary from opposing nozzle flows without any flow-smoothing screens to opposing flows through porous plates. It is desirable to select designs that correspond best to the conditions treated in available codes for calculating reacting flows because this facilitates comparisons of experimental and computational results. The most convenient codes to use are for steady laminar flows with one-dimensional scalar fields, and they often impose rotational plug-flow conditions at the boundaries. Accuracies of axisymmetric counterflow flame measurements in experiments intended to conform to these conditions are estimated here for designs of large aspect ratios with straight-duct feed streams that have multiple-screen flow-smoothing exits. Causes of departures from assumptions underlying computational programs are addressed by methods that involve theoretical analysis, experimental measurement, and axisymmetric computation. It is concluded that experimental results would not be expected to differ from predictions made with plug-flow boundary conditions by more than five percent for properly designed counterflow experiments of this straight-duct, multiple-screen type.