We study the isothermal delta' (Al3Li) precipitation kinetics in an Al-2.3 wt% Li-0.1 wt% Zr alloy by means of the measurement of the thermoelectric power (TEP) in the temperature range between 120 and 180 A degrees C. We obtain that the nucleation-and-growth stage of delta' precipitation reaction can be well described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) relation. This result suggests that the JMAK relation provides a good description of the impingement effect of growing delta' precipitates where interactions mainly occur on neighbouring precipitates. The activation energy associated to the nucleation-and-growth stage calculated from the JMAK fit is 52 +/- A 1 kJ/mol. The small activation energy obtained is ascribed to the presence of a large amount of excess vacancies quenched-in from the ageing temperature, inherent to the experimental conditions of the measurement of the TEP, reducing the activation energy to a value close to the vacancy migration energy in aluminium (45-65 kJ/mol). The Avrami exponent of this stage ranges between 1.5 and 1.65. These kinetic parameters indicate that delta' particles grow via a three-dimensional vacancy migration-controlled mechanism.