The prediction of the autoignition delay time of fresh fuel in hot air is studied. This problem, encountered in many combustion systems, is in practice often calculated using a simple method, hereafter called homogeneous mixing ignition (HMI). This method, however, neglects transport effects and calculates local instead of global ignition properties. A second method is therefore studied called linear mixing ignition (LMI) and consists of the direct simulation of a one-dimensional mixing layer. This method is tested in a hydrogen and methane fuel configuration using detailed and reduced chemistry mechanisms. The results indicate that molecular transport increases the ignition delay for the methane fuel by about a factor three, but this effect is compensated in the hydrogen case by the small Lewis number of the fuel. Such detailed information is not provided by the HMI method and the LMI approach appears, therefore, as a minimum requirement for the correct estimation of the ignition delay time.