The optimization of Ni phytoextraction processes needs further understanding od the relationships between Ni availability in soils an its absorption by plant roots. As a first step, the influence of the different Ni bearing phases and Ni availability has been partially established as well as its consequences on the tranfer of the metal to the shoots of hyperaccumulator plants (Massoura, 2004 ; Echevarria et al., 2006). Moreover, the unlimited metal uptake, as well as the release of organic ligands (citrate or malate) by several hyperaccumulator species (Jones et al., 1996) could accelerate the weathering process of Ni-bearing phases in the rhizosphere. This would lead to an increase of the bioavailable pool of metals. Therefore, a complete study was undertaken to understand i) the origin and nature of bioavailable forms of Ni, ii) the weathering processes of Ni bearing minerals and their kinetics and iii) the impact of hyperaccumulator plants on the evolution of Ni bioavailability in soils through the exudate-mediated weathering of bearing minerals. We selected three Ni-bearing minerals reflecting different ultramafic pedological contexts and also showing increasing degree of availability: i) a synthetic nickeliferrous goethite (oxi-hydroxide) Ą(Ni, Fe) OOH with 1.6 % of Ni ; ii) chrysotile (fibrous serpentine, phyllosilicate) (Mg, Fe, Ni)3 SiO5 (OH)4 with 0.4 % of Ni ; iii. Nickeliferrous smectite (Si, Al)4 (Al, Fe, Mg, Ni, Cr)2 O10 (OH)2 with 2.3 % of Ni. We also selected three different Ni hyperaccumulator plants from two different climatic areas: Thlaspi caerulescens (France), Alyssum murale (Albania) and Leptoplax emarginata (Greece). To answer these questions we used two approaches. The first one is based on the observation of on-site systems. We monitored the weathering of purified bearing phases placed in nylon bags within ultramafic soils at 10 cm below surface in temperate (France), mediterranean (Albania) and tropical (Brazil) regions under or without a hyperaccumulator plant cover. The second approach consisted in a laboratory experiment. We grew the three hyperaccumulator plants and an non-hyperaccumulator species (Aurinia saxatile) for five months in rhizoboxes in which the same nylon bags were placed in an acidic quartz silt soil showing very poor Ni retention ability. Plants were harvested and fresh samples were ground in liquid nitrogen for the determination of Ni speciation with EXAFS (Beam 16.5 at Daresbury Synchrotron). Thin sections of the rhizosphere of each plant-mineral combination were made and analyzed through Scanning electron microscopy and micro X-ray absorption spectroscopy (at Villigen Synchrotron) for precise location of Ni from the mineral to the inside of the root. Finally, Ni availability in the minerals and the different parts of the soil surrounding the mineral bags was monitored with Isotopic Exchange Kinetics (Echevarria et al., 1998). Speciation of Ni in plants seem to be quite even among plant parts and plant species and probably close to Ni-citrate. It is probable that citrate is released by those plants in their rhizosphere to enhance Ni availability in soils when poorly accessible (Boominathan et al., 2003). Other results are under acquisition.