Earlier bud-burst, earlier flowering and delayed autumn leaf fall have been noted in recent years and attributed to global warming.  With further warming, these trends are likely to accelerate.  At present, it is assumed that only temperature and day-length have a significant effect on plant phenology.  However, rising [CO₂] and [O₃] have profound direct effects on the physiology of plants.  This study uses soybean as a model plant to test the hypothesis that phenology is unaffected by [CO₂] and/or [O₃] in the field.  Well-defined developmental stages, genetic uniformity and the unique FACE field facility at University of Illinois Urbana presented an ideal situation to test this hypothesis.  Soybeans were grown at the UIUC SoyFACE research facility, where replicated plots were exposed to elevated levels of [CO₂], [O₃] or combined atmospheric treatments.  Phenological measurements were taken every two to three days throughout the growing season.  Exposure to elevated [CO₂], and to combined elevation of [CO₂] and [O₃], significantly delayed progression through all reproductive phenological stages in each of four years.  Elevated [O₃] did not alter development, except by accelerating senescence of the canopy.  Parallel observations of delayed onset of leaf senescence in elevated [CO₂], have been observed in FACE experiments with trees in Wisconsin (Karnosky Pers. Comm.) and Italy (Taylor, Pers. Comm.).  Together these data suggest that the frequently reported postponed Northern Hemisphere leaf fall is at least, in part, a direct result of rising [CO₂] and not solely a result of warmer, autumn weather.