The time of, and location where ignition first occurs in a diesel fuel spray were investigated in a rapid compression machine (RCM) using the two–dimensional techniques of silicone oil particle scattering imaging (SSI), and the planar laser induced fluorescence (LIF) of formaldehyde. Formaldehyde has been hypothesized to be one of the stable intermediate species marking the start of oxidation reactions in a transient spray under compression ignition conditions. In this study, the LIF images of the formaldehyde formed in a diesel fuel spray during ignition process have been successfully obtained for the first time by exciting formaldehyde with the 3rd harmonic of the Nd:YAG laser.SSI images of the vaporizing spray, and the LIF images of formaldehyde were obtained together with the corresponding time record of combustion chamber pressures at initial ambient temperatures ranging from 580 K to 790 K. Ignition delays measured by the two new techniques and by the conventional combustion chamber pressure history curve method were compared. It was found that both the time when the first local heat release is detected by the SSI method and the time when the first fluorescence is detected by the LIF technique are practically identical to the time when the total heat release due to the low temperature oxidation reactions equals to the heat absorption by fuel vaporization in the spray. The formaldehyde level rises steadily until the high temperature reaction phase of diesel spray ignition. At the start of this ‘hot ignition’ phase, the formaldehyde concentration falls rapidly, thus signaling the end of the low temperature ignition phase. Increases in the initial ambient temperatures advance the time when hot ignition starts. The first hot ignition occurs in the periphery of spray head at ambient temperatures between 580 K to 660 K. When the ambient temperature is increased up to 790 K, the position of first ignition moves to the central region of spray head. In addition, at initial ambient temperatures between 580 K to 660 K, the intensity of the first formaldehyde LIF signal remains at an almost constant level. However, it was observed that at initial ambient temperatures above 660 K the first LIF signal intensity increases by as much as nine times.