Autoignition in an unstrained, laminar mixing layer of methanol/air is investigated using detailed reaction mechanism and full multicomponent mass diffusion formulation. The temperature of the fuel stream is varied from 400 K to 1200 K, whereas the oxidizer stream is held at a fixed temperature of 1200 K. The calculations are performed for pressure p = 1 bar. Transient evolution of the autoignition kernel from initial partially premixed flame structures to final diffusion flame is demonstrated. The flame structures have been analysed for individual heat release rates. For equal fuel and oxidizer stream temperatures (1200 K), heat release in extremely fuel rich locations (with mixture fraction values up to 0.8) is found. A transient triple flame structure (two deflagrations and, one diffusion flame) is shown to exist even in cases when the temperature difference between the two streams is large. The heat release rates in the deflagrations depend on the temperatures of the two streams. When compared with the surviving diffusion flame, the heat release rate in the short-lived deflagrations is one to two orders of magnitude higher. It is shown that increasing the fuel stream temperature also decrease the ignition delay time in the mixing layer.