Under high pressures and temperatures, hydrocarbons demix forming separate carbon- and hydrogen-rich phases. This gives rise to diamond formation within icy planets affecting their internal dynamics and has implications for inertial fusion energy, where demixing in plastic ablators can seed implosion instabilities. Despite the widespread importance of this process, it remains poorly understood, with existing studies finding huge discrepancies in diamond formation conditions and timescales from 15 to 140 GPa. Moreover, there is limited data on the effect of composition. Here we report time-resolved x-ray heating of statically compressed hydrocarbons of various compositions and map the effects of pressure and composition on demixing timescales. Carbon-rich hydrocarbons with C:H ratios of 1:1 or greater form diamond on timescales of order $10\mu \mathrm{s}$ from 20 to 80 GPa, while hydrogen-rich ones take longer. This likely arises from the relative reactivities of saturated and unsaturated hydrocarbons and may support the existence of a theorized plateau in the chemical potential of hydrogen-rich warm dense carbon-hydrogen mixtures.

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• High-pressure studies
• Diamond
• Free-electron lasers
• Planets & planetary systems
• Precipitation
• Pressure techniques