Increasing the L-ascorbate (vitamin C) content of crops could in principle involve promoting its biosynthesis or inhibiting its degradation. Recent progress has revealed biosynthetic pathways for ascorbate(1-3), but the degradative pathways remain unclear. The elucidation of such pathways could promote an understanding of the roles of ascorbate in plants(4), and especially of the intriguing positive correlation between growth rate and ascorbate oxidase(5,6) (or its products(7)). In some plants (Vitaceae), ascorbate is degraded via L-idonate to L-threarate (L-tartrate), with the latter arising from carbons 1-4 of ascorbate(3,8-11). In most plants, however (including Vitaceae)(11), ascorbate degradation can occur via dehydroascorbate, yielding oxalate(12) plus L-threonate, with the latter from carbons 3-6 of ascorbate(3,10,13). The metabolic steps between ascorbate and oxalate/L-threonate, and their subcellular location, were unknown. Here we show that this pathway operates extracellularly in cultured Rosa cells, proceeds via several novel intermediates including 4-O-oxalyl-L-threonate, and involves at least one new enzyme activity. The pathway can also operate non-enzymatically, potentially accounting for vitamin losses during cooking. Several steps in the pathway may generate peroxide; this may contribute to the role of ascorbate as a pro-oxidant(14,15) that is potentially capable of loosening the plant cell wall and/or triggering an oxidative burst.