Petrological constraints on the timescales of pre-eruptive crystal storage and magma degassing provide an important framework for the interpretation of seismic, geodetic and gas monitoring data in volcanically active regions. We have used Fe–Mg diffusion chronometry in 86 olivine macrocrysts from the AD 1783–1784 Laki eruption on Iceland's Eastern Volcanic Zone to characterise timescales of crystal storage and transport in the lead-up to this eruption. The majority of these olivines have core compositions of Fo<76, and rim compositions in the range Fo69–Fo74 that are close to equilibrium with the Laki melt. Diffusion modelling using the greyscale intensity of backscattered electron images as a proxy for olivine composition reveals that the most probable Fe–Mg diffusion timescale for Laki olivines is 7.8 days, which reflects the characteristic olivine residence time in the carrier melt prior to eruption. A small population of Fo>81 olivines record Fe–Mg diffusion timescales of ∼124 days; these crystals are likely to have formed in mid-crustal magma chambers, been transferred to storage at shallower levels and then entrained into the Laki melt prior to eruption. Typical Fe–Mg diffusion timescales of 6–10 days are shorter than the average time interval between discrete episodes of the Laki eruption, indicating variable or pulsed disaggregation of stored crystals into the carrier liquid prior to the onset of each episode. The diffusion timescales coincide with historical accounts of strong and frequent earthquakes in southeast Iceland, which we interpret as being associated with mush disaggregation related to melt withdrawal and the initiation of dyke propagation from a crustal magma reservoir at ∼6±3 km depth to the surface. We calculate pre-eruptive CO2 fluxes of 2–6 Mt d−1, assuming a pre-eruptive CO2 outgassing budget of 189.6 Mt for the Laki eruption and a constant rate of CO2 release in the 6–10 days preceding each eruptive episode. Our dataset indicates that petrological constraints on the timescales of magmatic processes occurring in the days leading up to historic eruptions may enhance our ability to forecast the onset of future large eruptions, both in Iceland and further afield. •Fe–Mg diffusion in olivine is used to characterise timescales of crystal storage and transport.•Olivines were entrained into their carrier liquid 6–10 days prior to the Laki eruption.•Diffusion timescales are comparable to historical records of pre-eruptive seismicity.•CO2 fluxes in the 6–10 days preceding eruption were 2–6 Mt/d.•CO2 flux monitoring may enhance accuracy of eruption forecasting in Iceland.