The paper reports the relative acidification potential of 10 N2-fixing grain legume species grown in nutrient solution for 35 days after nodule appearance. The legumes studied were pilosus (Lupinus pilosus Murr. P23342), yellow lupin (Lupinus luteus L. R1171), white lupin (Lupinus albus L. Kiev mutant), narrow-leafed lupin (Lupinus angustifolius L. Gungurru), faba bean (Vicia faba L. Fiord), field pea (Pisum sativum L. Dundale), grasspea (Lathyrus sativus L. S453), chickpea (Cicer arietinum L. T1587), common vetch (Vicia sativa L. Blanchefleur), and lentil (Lens culinaris Med. ILL6002). The species varied considerably in their acidifying ability; proton production ranged from 77 to 136 cmol/kg dry matter. Chickpea and narrow-leafed lupin had the largest acidification potential and field pea the least. The specific acidification (amount of protons produced per kg dry matter) was best correlated with concentrations of excess cations, ash alkalinity, and calcium in plant across the species but was not correlated with plant nitrogen concentration. Total ash alkalinity or total excess cations in shoots provided a good indicator for estimation of total proton production in these species, which were reliant on N2fixation for their source of N. The results have implications for the selection of legumes to be used in sustainable farming systems.