There has been a considerable amount of interest in negative thermal expansion (NTE) phases which are of importance, for example, in the manufacture of low or zero expansion composite materials. Cubic gamma-ZrMo(2)O(8) is of importance since it shows smooth NTE (alpha = -8 x 10(-6) K(-1) at 100 K) over a wide temperature range with no significant discontinuities as a function of temperature. The material has long been thought to be metastable at all temperatures and has previously only been produced under kinetic control by routes such as the careful dehydration of ZrMo(2)O(7)(OH)(2).2H(2)O. High-speed, high-temperature in situ diffraction studies carried out at beamline ID11 at the European Synchrotron Radiation Facility (ESRF) have shown that it is in fact possible to prepare gamma-ZrMo(2)O(8) directly from the constituent oxides in a narrow temperature window around 1400 K where it appears to be entropically stabilized. Under these conditions gamma-ZrMo(2)O(8) forms in a matter of seconds and can be quenched to room temperature. Full quantitative Rietveld analysis of diffraction patterns collected in as little as 0.25 s during the synthesis has allowed the reaction pathway to be followed.