The thermal and structural behavior of anhydrous milk fat (AMF) was studied by a technique that allowed simultaneous time-resolved synchrotron X-ray diffraction as a function of temperature (XRDT) and high sensitivity differential scanning calorimetry (DSC) to be carried out in the same apparatus from the same sample. In this paper (the first of a series), the less stable crystalline structures made by triacylglycerols (TG) of bulk AMF after its liquid quenching down to -8 degrees C are addressed The coexistence of two lamellar structures characterized by sharp long spacing reflections corresponding to well-defined 3L (70 A) and 2L (47 A) longitudinal stackings but broad short spacing lines related to poorly ordered hexagonal (alpha) lateral packing is shown for the first time, The bilayered structure was very unstable, since it disappeared during a 20-min isothermal recording. Simultaneous DSC and X-ray monitoring of AMF heating in the range -8, +50 degrees C at a rate of 2 degrees C/min allows the same sample to be followed on the evolution of these unstable forms to more stable varieties. The 3L stacking transforms into a new 2L crystalline structure characterized by broad LS reflections corresponding to a ill-defined 2L (37 A) longitudinal stacking but a more compact orthorhombic (beta') lateral packing. A delimitation of the domains of existence of the crystalline structures resulted from the comparison of detailed analysis of the evolutions of positions, intensities, and widths of X-ray peaks as a function of temperature to microcalorimetry recording.